Questions about working with fiberglass doors?
Trying to set a new energy efficient door into an old frame is a waste of time, from an energy standpoint. Better to install with a new weatherstripped frame and threshold. It can be done by someone that is familiar with such things, but is usually more expensive in time than replacing the whole frame.
To answer the question, there are limitations to cutting fiberglass doors, which vary by manufacturer. And it depends on how much you are cutting. Most doors have wood cores or internal frames. Some have more wood than others, which makes trimming them different. If you cut past the wood frame, it needs to be reinstalled(gets complicated). Better not to try to modify them.