Yes it is possible and these folks here have found out a way to do it:
http://labs.qi-ideas.com/2007/12/25/using-flex-compiled-code-within-flash/
Simple. Logical of course the classes are available because of the way flash works. I was going to spend two days rewriting the rpc classes!! Very interesting stuff they’re doing over there….
Compiling Flex Classes in Flash CS3