I avoid using any processing of the source data in a data source - it is not transparent, the users have no control and you are losing the audit trail back to the source. None of these might be a problem for you though. I would try to handle the requirement through Transformation Rules - they have similar functionality, are transparent, can handle exceptions by Entity etc.
You will probably have a UD dimension that maps everything to a single Cube member i.e. there is a single Mask map that says * to abc. (If you do not have any empty UDs, add an attribute dimension to the Cube and use that instead). Then, on the Data Source I would bring in whatever will make the mapping easier: concatenate the two source fields or the final 8 characters of the Account. Then in the maps use a Complex Expression to reference that this new piece of source data.
