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.
Europe, Middle East and Africa