Hi Nirav,
I have also faced same issue. client asked me to create new column in ME51N to know the total stock.With the help of ABAPer I have done it..But faced lot of issues.
Finally with the help of ABAPer in ME55-During release of purchase requisition, created new column as "Total Stock".
By seeing total stock user can release the purchase requisition. or else he can release it later.
You can create new field as "ZLABST". Surely you can achieve this with the help of Good ABAPer..
Regards
Subbu.