Enhancing Data Sources with Custom Fields

We currently are extracting data from R/3 to BW from billing document header (2LIS_13_VDHDR) and item data sources (2LIS_13VDITM). These data sources are based on VBRK and VBRP tables.

The tables have been enhanced with some custom fields. These have to be added to the data source and brought over to BW. How do i add these fields to the corresponding data sources and how do i populate them. Please advise.

Thanks

Rashmi.