No. PTO is a company benefit, generally. The FMLA is a federal job-protection leave law. Employees are entitled to use FMLA leave in certain situations.
Assuming an employee is FMLA-eligible (and the condition qualifies), time off should be designated as such. PTO can run concurrently with FMLA in most cases. Making employees use PTO first, however, could lead to an FMLA interference claim.