Yes, it can be done. Even by a Do-It-Yourselfer.
Modern brazing alloys allow this. There is one new alloy in particular worth mentioning. Here is a link to the thread where I just got done repairing my MTB frame. So, for those who think it's imposible to effectively and safely repiar a frame easily, or even at all, you may change your view after seeing this.
I only posted this for others who don't frequent the framebuilders forum to see.