There is not a way specific to Fountain Mode to include additional files. The author’s advice is to leverage the power of other Unix tools. One way is to use a Makefile, like this:
DRAFT = Memoryhouse.fountain INCLUDE += prologue.fountain INCLUDE += act_1.fountain INCLUDE += act_2.fountain INCLUDE += act_3.fountain INCLUDE += denouement.fountain draft: cat $(INCLUDE) > $(DRAFT)
A more complicated way could use M4:
title: __TITLE__ credit: written by author: __AUTHOR__ m4_include(prologue.fountain) m4_include(act_1.fountain) m4_include(act_2.fountain) m4_include(act_3.fountain) m4_include(denouement.fountain)