How do I create a user variable that uses another user variable as part of the definition?
Unless FM10 is dramatically different than earlier releases, you can't.
You're limited to the Building Blocks provided in the dialog.
There are work-arounds, possibly including your own FDK extensions, but a simple hack might be to build paragraphs from Variables on a Reference Page. Then other such paras could use those paras as needed to create complex Var expressions. This does have a minor hypertext side effect when finally Xref'd in Body text.
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