The short answer to this is that it isn't possible without radically restructuring the Frankish army (probably going back to Martel's time if not earlier). Why? Logistics. Charlemagne can't get his army to Andalusia in any sort of fighting-capable state. And even if he did it wouldn't be worth it.
In Charlemagne's reign the general practise for the Frankish army was to have large magazines in each major city, filled with food and other supplies that would be needed. If the army was travelling more than a few days' marching, it would go to one town, then pick up enough supplies to get it to the next, and then collect more there. Repeat as many times as necessary to get to the hostile frontier. This method is very good for responding to internal threats (rebels) or external raiders (Saxons), which is the type of fighting most of the Franks had done since the 6th century, as it allows a force to be rapidly organised and moved around the country without impacting the locals, but isn't well suited to large scale offensive action as it requires a massive supply train. For an army of 40,000 men in enemy territory, around 500 carts' worth of food is needed to sustain it for a week. In the Italian campaign a big point was made of not taking food from the locals (or looting the land at all), as Charlemagne feared that they would rise up and fight him. If the Italians got to the Frankish supply lines, the army is soon defeated (or more accurately, forced to retreat).
Italy is really the only major campaign where Charlemagne advanced any sort of distance into enemy territory. The first stage of it (pre-siege of Pavia) was covered by the aforementioned supply train and non-interference policy. During the time of the siege, the Franks had another force, headed towards Verona, but here the Pope was supplying the army with food (a fact that the Pope would spend a lot of time reminding Charlemagne of later). Otherwise, Pavia is only about 100km (around a week's march) from the border. In the Saxon campaigns (at least 772, 775 and 776 when the Franks had minimal infrastructure in the area), they only advanced around that far as well. Barcelona is no further from the border either. Narbonne -> Barcelona is also a similar distance to Barcelona -> Zaragoza. So a week's march is likely to have been the limit of his capabilities (raids may have reached further, but raids are not huge armies).
Even if he can march to Cordoba, how will he keep them supplied? He's already had to build more carts and transport thousands of extra animals over the Pyrenees, so even more carts will eventually get unwieldy. 40,000 is a rather large army for the time, supplying it ten times over (so it can last two months instead of a week) is reaching into the absurd. If he takes food from the local farmers, they'll join the Saracen army or raid his supply train. If he doesn't, he starves (and the Muslims in Andalusia may well resist him anyway). Perhaps not so much a problem if the Umayyad leader capitulates immediately, but if he forces the Franks to a siege, Charlemagne isn't going to make it work. The Muslims would be well aware of this too.
Not to mention that by 800 Charlemagne was getting quite a bit older than when he fought the Lombards and Saxons. The Royal Frankish Annals mention him spending more and more time at Aachen, in contrast to earlier years where he is often seen at various different palaces, so it is quite possible he simply couldn't be bothered committing to another major campaign (in addition to the quite demanding work of being a king in the first place).
Assuming that the circumstances would have allowed a large scale campaign in Eastern Spain and Charlemagne manages to conquer most of Hispania (maybe also puppetizing the weak Kingdom of Asturias), crushing the Saracen hegemony in the Iberian peninsula, which consequences would have had the incorporation of 'Gothia' in the Carolingian Empire after the death of Charlemagne?
Handwaving all the issues I just listed for a minute, let's say that he does make it to Cordoba, the Umayyads throw open the gates and he's now king of Spain. The population of his empire has now doubled again*. Again? Well it kind of doubled (compared to Pippin's day) when he took over Italy, Saxony and Bavaria. So he's now ruling between three and four times the amount of people his already busy father did. Why is this a concern? The main way of governing was to pretty much ride around to the different parts of the empire and tell people to do what he wanted. Not always personally of course, but if he's not getting on a horse, someone else is, which means a lot more 'someone else's have to be hired. However it is done, it won't be governed nearly as effectively as if he had fewer subjects to manage. And the entire empire suffers for it.
Finally, we come to when he dies. Yay, Louis is king. Maybe one of Louis' brothers lived. If that's the case, the division is probably Spain and half of France to one, the other half and Italy to the other (or Spain/France/Italy if there's three alive). However it is divided, it will still suffer the age-old problem of partition inheritance. Except there's butterflies so it's too hard to say where the chips might fall thirty years down the track.
*Using numbers estimating for the year 1000, I couldn't find anything earlier but the ratio wouldn't have drastically changed.
- BNC