Hi Sushant
I thing you are talking about Cost Center Balance showing under-absorbed.
You are right, if these cost are not allocated to production cost center then it will not flow to production activity cost during activity price calculation and remain in cost center.
You can allocate these costs through cost center allocation :
1. Through Assessment cycle
2. Through Distribution Cyle
You have to create required cycle for plan & actual separately
KSU7 for plan assess cycle
KSV7 for plan dist cycle
KSU1 Actual assess cycle
KSV1 Act dist cycle
Kamal