I've already brought up the old criminal investigation triad of Means, Opportunity, and Motive in the Vinland thread so I'll repeat Grey Wolf's cogent question again: Why heaven's name would Germany do this?
Let's look at the issue with means, opportunity, and motive firmly in mind.
It's arguable that Germany doesn't even have the means necessary to lift 100K troops to North American and keep them supplied. Furthermore, Germany's navy isn't anywhere near superior to the US' navy at the time and may actually be worse in some aspect.
As for opportunity, given the armed camp nature of Europe and Britain's then current war in South Africa, Germany loading 100K troops aboard ocean liners - something which cannot be hidden - is going to raise an awful lot of questions in an awful lot of European capitals. Germany isn't going to be able to sneak an army corps across the Atlantic and, because her explanations of just where that army corps is headed must be believed by the various European powers who will be asking very pointed questions, the US is going to learn about the invasion force before it leaves Germany harbors.
Finally, any motive is wholly lacking. Yes, Germany wanted a piece of the Spanish empire and, yes, Germany engaged in little games of chicken in the hopes it could buffalo or scare the US into concessions. There is no way in hell, however that, absent wholesale changes to the international situation of the time, Germany would ever seriously consider invading the US.
The staff study Conroy wrote his silly book about was just an exercise, just as general staffs the world over "plan" for silly shit like zombie plagues, alien invasions, or attacks on their best friends. The study was also done to placate Wilhelm II, a man whose most recent biography makes a strong case for suffering brain damage during delivery.