Raju,
You need to find out what is the search help that is being used for the standard field. Once you determine this, you need to create a custom search help and then append this to the standard search help.
In order find the standard search help that is being used, activate debugging and then hit F4. Add a break point at the FM F4_INTERNAL_TABLE_VALUEREQUEST. Check the parameters and you will know what is the search help that is being called.
Thanks,
Vikram.M