No idea if there is a way to make them pitch longer. The question from me is why would you want them to? Too me it's about shaking off rust throwing the fewest innings/pitches possible to avoid injury. If it's about looking at minor leaguers and whether they are ready to make the jump? IMHO a 4 inning start is as good as 5 or more. YMMV.
Please don't take this as a criticism of how you play, or want to play.
It's just me wondering what your thought process is?
Thanks.