I know there has been several people including me that want a way to use PostNuke and YaBB SE together. I have found a possible way that someone who has good knowledge of how YaBB SE works could do it fairly easily with out modifying YaBB SE.
One of the lead developers of PostNuke (Neo) has been working on a bridging method called Embedded PN that will allow virtually any PHP Program work within PN. To make it work for a program you build a "Bridge" with this script he is working on. He has been able to do succesfully with phpBB (which you can view at his beta site). Also modders from other Bulleting Board systems such as VBulletin Board have successfully been able to use it with theirs. I think it would be great if someone could do it with YaBB SE.
Anyway here is the link if you want to check it out and get more info.
http://beta.samstech.net/