Variable formula (in expression)

[NXLars]

I've been searching for, and trying to, determine wether it's possible to use variable formulas in NX(6).

The thing I want to do, if possible, is to use an expression to control another formula. This is to allow the modification of one expression to control a number of other expressions.

For example:
I'm working with a template to generate stage dependent components for a turbine. For each stage I'm using interpart link to "collect" the data that defines that stage. An expression can look like:
rotor_diameter = "data_prt"::ST11_rotor_diameter
where ST11 tells me which is the current stage.

What I want to do, if possible, is to set the prefix "ST11" as an expression so that I can change all expressions containing that prefix by changing the prefix expression instead of changing every expression containing STxx...

stage=ST11
rotor_diameter = "data_prt"::"stage"_rotor_diameter => "data_prt"::ST11_rotor_diameter
stator_diameter = "data_prt"::"stage"_stator_diameter => "data_prt"::ST11_stator_diameter

While
stage=ST12
rotor_diameter = "data_prt"::"stage"_rotor_diameter => "data_prt"::ST12_rotor_diameter
stator_diameter = "data_prt"::"stage"_stator_diameter => "data_prt"::ST12_stator_diameter


I've succeded in creating this as a string value, but I can't find a way to use the string value as formula in the next expression...
Are there any way aroud this inside of NX, or do I need to do this on the "outside"?

Best regards
/Lars