Resource with non-integer index

I have an existing database that uses text values like "T123456" for employee identification. I want to assign planner items with employees as resources but the resource index apparently can only be an integer. Is there a way to work around this without modifying the existing database?

The purpose of the DBxxxSource.ResourceMap is to map such resource values to an integer resource position.
Alternatively, you can also use the event DBxxxSource.OnResourceToPosition