Extra build tools

I am trying to build a flatpak using the kde sdk and runtime. My Qt program builds and runs correctly, but when I also try to build the documentation that is packaged with the program I need tools that are not available. I am building on Fedora 32 with core18. I need the following tools:
po4a
xsltproc
fop
docbook-xsl
docbook-xml

How do I add these extra build tools to my flatpak build?

I am building on Fedora 32 with core18.

core18 is a snap thing, it has nothing to do with flatpak.

po4a
xsltproc
fop
docbook-xsl
docbook-xml

xsltproc and docbook are included, just search for them under:

/var/lib/flatpak/runtime/org.kde.Sdk/x86_64/5.14/active/files

For the others: Add them as regular modules and clean them up completely (cleanup: "*"), since they are only needed at compile time.