Pentable/Text Substitutions

Wonder if we can code a text substitution to substitute another text substitution.

$SHTNO$ = "_FILES_"        => If PP01.dgn is open, this SHTNO = PP01.dgn

PP01.dgn = "10"                    => then, SHTNO = 10

The idea is that we can use the same $SHTNO$ text substitute for every cutsheet file, say, PP01.dgn, PP02.dgn, ..., PPn.dgn. When a cutsheet file is opened, SHTNO = PP01.dgn or PP02.dgn and so on. Then if we have a pentable that assigns an actual sheet number, say, PP01.dgn = 10, PP02.dgn=11, and so on, the actual sheet number will be stamped on each cutsheet.

Hope you can help. Thanks!