Johnmeyer wrote something that can help you get started.
In his guide he is talking about playing several different MPEG files (titlesets) one after each other.
End Actions Using IFOEdit

If MPEGs in your submenu are not in the main menu you could simply edit the post command in the VTS_PGCITI for that VTS IFO file to call the
PGC of your submenu.
I use a program called MENUEDIT to examine which PGC the menu uses for the submenu by looking at the actual code of the submenu button.
The Post command should look like:CallSS VGM PGC: 5 Resume cell 1

If they are in both or more than one submenu then you need to test a GPREG to know which PGC to return to.