I don't think there is a way to access the built-in sheet to make changes to it, so you would probably be better off temporarily making changes to the Hero during printing from a script.
(Theoretically, you might be able to make your own output dossier that just adds additional sheets/content to the built-in version if you can find out or guess the unique ids for the sheets or layouts used in the built-in version. But you would still be very limited in terms of changes you could make to the original content.)
Assuming that you are trying to fix the Grab output bug, it looks like the built-in MM3E sheet won't output certain picks in the Offense section if they have a Defense.? tag.
You can find out if the script is running during the output process (dossiers, text stat blocks, etc) by checking the value of state.isoutput, and maybe then remove the Defense.? tags after everything else has already been calculated?
Something like the following in an adjustment maybe, either targeting Grab specifically or iterating through all added weapons with a Defense.? tag?
Code:
perform hero.childfound[wpGrab].setfocus
done if (state.isfocus = 0)
if (state.isoutput <> 0) then
~ make whatever changes
perform focus.delete[Defense.?]
focus.field[atkSpecial].text = "vs Spec"
endif