A puppet country will allow you to take their armies so you have direct control over them. That is the only way to force other countries to help you on the front line. You can tell allies to not protect your borders but they may still keep them home anyway.

Wow, thats too bad, I thought I could use ROM/HUN forces to help out without having to accept their exp forces :(

Italy sends troops to help invade Russia so go figure.

an alternative is to take control of ROM using the console command tag rom and send the units to yourself als expeditionary forces. Just make sure the game is paused when you do this as otherwise the ai will take control of your germany and mess everything up big time.

Agreed on tag-switching to other countries to clean up their militaries. Especially in really long-running games. I routinely switch into my puppets and faction members to delete trash divisions, redesign their templates, queue up useful new templates, send myself expeditionary forces, and set sane conscription laws. This gives your main nation usable reinforcements and also keeps the AI division spam down.