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Have you tried Designated Area under Engineering?
I don't have Creo 7.0 to test, but I recall that such tool also exists in Creo Simulate?
All ways previously suggested (Divide, Designated Area, Split in Creo9, SurfRegion in CreoSim...) are possible inside Creo, but we do not have control on how do other software behave when Creo files are imported.
