if staff volunteered to pick up a shift is it not an issue with Fair Labor?

We have PT employees who can work up to but not including 30 hrs a week - our FT employees are any staff that work 30 or more hours per week. If we have PT employees that are volunteering to pick up hours - is that then an issue? Do we need to get something in writing from them that they volunteer on a regular basis to pick up hours and acknowledge that they have not been put on the schedule against their will by the employer? Is there a certain amount of hours that employees can work over our PT classification without having to be reclassified as FT? Could you also give me a reference link? I searched the DOL website and fact sheets but was unable to locate anything on this.