Warning: This is a massive bodge, however i could not see any other method for doing it.

Instructions:

• Goto http://bl.ocks.org/veggielane/raw/449293ab850608d73a93/
• Change the String Length, String Param and Output Param.
  • The String Param is the Input parameter name.
• Copy all the relation text.
• Paste it into the relations dialog in Creo.

The source can be viewed: https://gist.github.com/veggielane/449293ab850608d73a93

Caveats:

• Only works on a known length string.
• Only works on whole numbers (but can output to a Real Number parameter).
• To output to an Integer create the output parameter before adding the relation (it defaults to real).
• For long strings the relation can get quite large, and i am unsure about the performance hit.