Substitutions - field does not appear in list of 'substitutable fields'

Question

Specifically, I would like to use a exit substitution to replace the content of the Structure PRPS-POSID field.
However, I cannot seem to use a simple substitution rule (since PRPS-POSID does not appear as a substitutable field in the PRPS list of fields that pops up when creating a 'Step').  Most of the PRPS field are availed as 'substitutable fields' but not POSID.
The user exit approach is proving difficult since I cannot add the following code unless I put in a data or tables statement (which is NOT recommended).
If proj-PSPID(7)  <> prps-POSID(7) .
  prps-posiD(7) = PROJ-PSPID(7) .
ENDIF.
Please help!

Answer 1

Hello Shrikant;
The requirement is to ensure that the WBS element numbering (prps-posid(7) ) will always match that of the Project Definition (proj-pspid(7) ).  This numbering rule is required to comply with the Fund Centre numbering since the fund center number utilizes the Project Definition number as an identifier ( I will email you some screen prints).
If there is a discrepency between the WBS element numbering (prps-posid(7) ) and the Project Definition (proj-pspid(7) ) number, the requirement is to assign Project Definition (proj-pspid(7) to the WBS element numbering (prps-posid(7) .  The transaction used to call the substitution exit is CJ20N.
A big thanks for your comments so far.
Peter Forster.