Michael,
That is the standard behavior of the extractor, as we all know WRTTP is used internally for the value type calculation may be that is the reason the field is not available to be pulled in to BW.
Anyways you can use the VTYPE field in your reporting and restrict it to 10 for Actuals and 20 for Plan, which has the WRTTP values as 4 and 1 respectively.
NOTE : There will be Statistical Actual postings too with VTYPE = 10 respective value of WRTTP is 11,but you can differentiate this with the VSTAT field which will have value 1 for WRTTP 11.
Still if you want to bring that field , you can work with the ABAPers to see how you can modify the code,but before that raise an OSS message with SAP and get their suggestions.
Is there a specific reason you want to bring that field separately.
Hope this helps...