SU:s are pulled to the F.Z. via the Assign interface. The F.Z: can pull from HQ:s in their line of command. Each SU will cost an AP.
Best way is most likely to first assign F.Z. to the Corps with the construction SU:s, pull said SU:s and then re-assign the F.Z. to OKH. The last step is to free up the Command of the Corps and, since digging above lvl 3 takes an ADMIN roll, to get a good leader in charge (OKH usually have a good ADMIN leader, but few Corps have).
It also takes a huge amount of supply! 10k to go from 3->4 and then 10k more to get to 4 + 10%. Huge is relative, but every point must be transported to the hex, probably by trucks that will be damaged/destroyed.
In all, lvl 4 should probably be avoided during '41! You get 10 lvl 3 forts for every lvl 4 (except for the extra AP, of course).
Edit: There is a limit to the distance that an SU can be pulled from an HQ. I think it varies with the level of the HQ, but have not tested it. You might want to do some tests in sandbox mode before going live.
< Message edited by gingerbread -- 8/24/2012 3:19:33 AM >