How to customize cost management engine in R12

Good day Experts,

We have a scenario where we need to customize cost management engine to calculate the periodic average rate based on YTD purchasing value instead of PTD purchasing value, however, this rate will be applied on incremental basis therefore we cannot use a single inventory period for whole year to calculate the periodic average cost. Kindly follow the table below for the complete understanding of scenario:

   

      

		Purchases
	Qty	Rate	Value	Cost for issuance	Calculation
Opening	10	10	100	-
Jan	10	20	200	15	(100+200)/(10+10)
Feb	10	30	300	20	(100+200+300)/(10+10+10)
Mar	10	40	400	25	(100+200+300+400)/(10+10+10+10)

We need to calculate the cost for issuance as per the example table provided above. As far as I know this cannot be achieved through standard costing engine therefore we want to customize the cost engine to cater our requirement. Pls provide comments whether it is possible to customize cost engine or not? and if there is any other work around for this issue.