Short answer: Not when the absence is FMLA leave paid by a disability plan. Because that leave is not unpaid, neither the employer nor the employee can require PTO to run alongside it, though they may agree to use PTO to top up the benefit where state law permits.
FMLA lets an employer require accrued paid leave to run at the same time as unpaid FMLA leave. But 29 CFR 825.207(d) says that because leave under a disability benefit plan is not unpaid, the substitution provision does not apply, and neither the employee nor the employer may require substitution of paid leave. The same rule applies to a workers’ comp absence (825.207(e)).
They can agree to have paid leave supplement the disability benefit, for example where the plan replaces two-thirds of salary and PTO makes up the rest. Put the top-up policy in writing.
Unpaid stretches are different. During the STD elimination period, or after STD ends, the leave is unpaid and the normal substitution rules apply again. Check the employer’s written leave policy as well.