There are several different ways to insert USFM which isn't in the paratext files. One option is to use the changes.txt file, but that gets a bit messy, with you needing to double every slash.
You could try using a hook, but the problem with hooks is that in hooks numbers are considered to be numbers, not things that might be part of a marker, i.e. \mt1 will be understood as \mt 1.
The most initially complex, but maybe eventually the easiest once you're used to it, is to make a 'trigger file'. If your project is called MYPROJ, and the configuration testing, then for a glossary the file would need to go into
{paratextDir}/MYPROJ/shared/ptxprint/testing/triggers
with the name A9GLOMYPROJ-testing.SFM.triggers
In that file, you can put one or more trigger entries, e.g. like this:
\AddTrigger GLObookstart-preverse
\mt1 ~
\zgap|8em \*
\zrule|width="0.5"\*
\zgap|1em \*
\mt1 GLOSSARY
\zgap|1em\*
\zrule|width="0.5"\*
\pb
\EndTrigger
The trigger entries can be anchored to any verse or \k keyword\k* entry, etc. much like figures (though to get the figure-like position you need to add a suffix "-preverse" as here), and should allow you to include any valid USFM. If you use no suffix, then the USFM is triggered after the triggering verse number, for verses, and possibly not at all for other locations.
Of course, you'd need to use 45ACT...SFM.triggers and ACTbookstart-preverse