Right now you can import precanned definitions of variables and expressions, import your frame variables and coordinate transforms and whatnot, but I believe you cannot maintain a library of importable procedures that use those expressions because you cannot store a procedure using a custom expression without also storing its definition, and import program will not allow a second import even if the definition is identical.
It seems so easy (he says): expressions are standalone, single-entity. If the xml for an imported definition exactly matches the xml for the existing one, discard the imported one and use the existing one.