Juhi,
Are you using Investment Measure? We did a similar conversion a few months back. We did the following steps, and it works fine:
1) Find the list of all open IO, and all expenses against that IO
2) Load expense to IO as a normal GL journal entry. You can use a BAPI or any custom upload program to load this JE
3) Then use KO88 or KO8G to settle to AuC.
You can not post directly to an AuC because it is tied to an investment measure. Therefore there is no upload program you can use. You have to go via the settlement process
I hope this helps. As we have done this process multiple times, if you need more information, please let me know
Thanks
Asheesh Bhatia