Modifications are no problem, as long as you keep these points in mind to retain the UL-B fire-rating:
Discuss your specific needs with a Terra Product Specialist.
Fire-rated doors are designed to be mounted flush with the wall, with the body of the pass-through extending into the unclassified room (see illustration).
Terra offers support brackets and hardware to ensure secure mounting (order separately). In most cases, a bracket kit provides ample support. Select support brackets for 36"D (914 mm) wall-mount models to brace the larger chamber. Two brackets are recommended for wide chambers (installed side-by-side on the wall).