Worker receiving disability benefits: Can we substitute unpaid FMLA for paid leave?

Q. My employee, who is a union member, has a nonwork-related injury that requires a six-week absence from work. In the meantime, she is receiving $300 per week through a union trust fund that provides her and other covered union members with short-term disability benefits. She also has requested and was placed on FMLA leave. We’d like her to substitute any unpaid FMLA leave with paid leave, which is our usual company policy when someone is out on FMLA leave. Is there a problem with doing this in this situation?