Yeah, you can build your own - it isn't trivial though...you can often find tubesets and lugsets on eBay that go reasonably. I used a bernzomatic brazing kit once upon a time to affix some cantilever brake bosses and it worked splendidly (like, the brakes never failed in traffic or a steep downhill or anything - success, IMHO). Brazing attachments onto an existing frame is immensely easier than building your own frame from scratch, however. We have several frame builders who often monitor this forum; maybe they'll give you their advice - you have to post pictures when you are done!