I am trying to figure out how to use an existing model of a hyroformed tube to create a split clamp with a matching internal form of the tube OD. Does this make sense?
You failed to state what version of Inventor you are using.
2010 multibody solids
or
2010 and earlier releases Derived Component derive body as Work Surfaces. F1
R10 doesn't have multibody solids (introduced in r2010)so you will have to research Derived Components. Very powerful and very robust. When you get something done attach the file(s) here if you get stuck.