Can a full-time Salaried employee take on in addition to their regular salaried position a part-time hourly position with the same employer?

Generally no. But the employer might allow it.

Are there any legal reasons (wage & Hour laws) why a full-time Salaried employee can not also take a part-time hourly position with the same employer?

Please don't start a new thread for a follow-up question.

I don't think there are any laws preventing this. But there are also nbo laws compelling an employer to allow it. Generally an employee is on the books once and is paid for the work they do. I doubt if any payroll system would allow for an employee to be listed multiple times.