yes, you can. If you go back for combat recourse and pass it,
and you will most likely to be post to a combat vocation, eg rp,
combat medic.
But if you chao keng, and keep failing your FFI for recourse, you
will stay in service vocation for your NSF life. But as NSmen, you
have a higher chance of being post to a combat unit. That is if
your pes is still A or B. But if your unit want you back as a
NSmen, you can have a chance of being a combat fit storeman. They
just need to write to CMPB to request it.
Just a sidenote: I once had a campmate who request to upgrade his
pes from E to B. Cause that is the only way he can post out of my
unit. And he got it. 