Hi Atif,
In note 577502 under restrictions you could find the following:
Restrictions
o A support of the user-defined fields in the mass maintenance of the customer master record (Transaction XD99) is not available.
In general you could do the following:
1st/ extend DEBMAS idoc
2nd/ Implement CUSTOMER_ADD_DATA_BI
3rd/ for ALE and batchinput: Make sure your implementation of CUSTOMER_ADD_DATA_CS is compatible with batchinput processing. This you can try and investigate easily with transaction SHDB. 4th/ Then if XD99 has to be in the loop, then refer also to note 414020.
I hope this helps.
Best Regards,
Tao