not automatic, but effective:
1 at start point of conditional code open fileref, say, myRef
2 push conditional code to myRef using the file statement MOD option in each subsequent data step
3 once a decision to use the conditional code is made, %include that fileref with code like[pre] data ;
if conditions = ready then do;
call execute( ' %include myRef /source2 ; ' ) ;
end ;
run ;[/pre] the point being that %include in open code will be executed unconditionally, so wrap call execute() around it to apply your conditions.
good luck
peterC