The short answer is, "no." An employee cannot terminate a disabled or injured employee just because their FMLA leave expires. At that point, the question becomes whether continuation of the leave beyond 12 weeks is a reasonable accommodation for a disability. In other words, if it doesn't pose an undue hardship on the employer, then the employee must be allowed sufficient time to recover and return to work, with or without further accommodations.
First, know the basic rule, all work performed in excess of 40 hours in one week is considered overtime and must be paid at one and a half times (1.5x) your regular rate or pay. Let's say your pay is $15 an hour and you work 50 hours in one week, you should be paid $22.50 for any work performed over 40 hours in a week. Thus, you would be owed $600 in regular pay (40 hours x 15) and $225 in overtime pay (10 hours x $22.5).