This is really three questions:
First, could it legally happen. Yes, but only under very specific circumstances. A judge could strip a citizen of his rights as part of a sentence of capitis deminutio, and that might specifically include sentence to gladiatorial combat (though those appear to have been quite rare). Prisoners of war could, if they were found to have dishonoured the Romanm arms, be sentenced to remain slaves after being returned home, though that is not something that IIRC ever happened in the Principate. Roman citizens could also self-sell into slavery, and in legal theory a father could sell his children. The latter was very much theoretical in the timeframe we are discussing, but it would stand up in court.
Second, could it practically happen. Sure, all the time. The Roman world didn't have a system of registration and identity documents. People were valuable merchandise. Kidnapping and enslaving were lucrative crimes. But in the case of a decorated general with social connections out the wazoo, I don't think so.
Third, could it conceivably be done. No. An emperor could kill a man like Maximus - they did it all the time - but not sell him into slavery. That goes against all social convention. Slaves are made of socially weak people: outsiders, the poor, the very young, the displaced. Not of senators.
First, could it legally happen. Yes, but only under very specific circumstances. A judge could strip a citizen of his rights as part of a sentence of capitis deminutio, and that might specifically include sentence to gladiatorial combat (though those appear to have been quite rare). Prisoners of war could, if they were found to have dishonoured the Romanm arms, be sentenced to remain slaves after being returned home, though that is not something that IIRC ever happened in the Principate. Roman citizens could also self-sell into slavery, and in legal theory a father could sell his children. The latter was very much theoretical in the timeframe we are discussing, but it would stand up in court.
Second, could it practically happen. Sure, all the time. The Roman world didn't have a system of registration and identity documents. People were valuable merchandise. Kidnapping and enslaving were lucrative crimes. But in the case of a decorated general with social connections out the wazoo, I don't think so.
Third, could it conceivably be done. No. An emperor could kill a man like Maximus - they did it all the time - but not sell him into slavery. That goes against all social convention. Slaves are made of socially weak people: outsiders, the poor, the very young, the displaced. Not of senators.