It might be possible to hook in some of the DB3 modules to DB2. It's a bit tricky because you'd have to go through COM, but it's possible in principle. Sunweaver, for instance, could pretty much drop right in without much effort. Blacklight would be more effort but still not too hard (the hardest part would be figuring out how to build up the data hierarchy for what to draw. Either XML or manipulating instances of classes. Probably XML (or something like it. Lua code isn't a terrible data format, actually, and a bit more compact than XML)).
Not a task I ever really want to do myself, but something I'd support.
Going the other way, pulling in DB2 code to DB3, is a different story. DB2 code is not really modular, and trying to extract out individual components from it would be self flagellation at best. It could be made modular, but that's a massive refactor, and I don't think it could be done without introducing numerous new bugs.