WBS element does have an conversion routine at the domain level. So sometimes you work with one format (8 digits) and sometimes with 24 digits.
You use FMs
CONVERSION_EXIT_ABPSP_INPUT
CONVERSION_EXIT_ABPSP_OUTPUT
to convert between values. Do your conversion before your select and do not forget to use a large enough data element for the field, which gets the 24-digits long value of the field.