Hi,
The best way might be to just populate the ALV control structure CONVEXIT field to control the same this way the Looping and extra processing in code(via FM usage) can be avoided. Also see if the output data type of the column is appropriate one used for WBS elements.
The domain PS_POSNR is the one which has the Conversion Exit built in. Just use it before using the ALV control structure CONVEXIT field by setting it as 'X'.
Cheers,
Arindam