Welcome to the site, and I'm glad to hear you find this site a worthwhile successor to your own community! I hope you have great success with it.
I'm not sure if there's much of a strategy beyond encouraging people to start using this site. Yes, if the participants already have a Yahoo! account, they will be able to it as their Open ID to log into this site (one less username/password they have to remember).
But we don't really have any provisions for importing content from other systems, nor would that really work particularly well. The question/answer format of a Stack Exchange site works somewhat differently from a traditional discussion-style Q&A, so the content does not lend itself to copying en masse. It's better to simply ask people to check out the site and start asking their questions… and to improve the answers with their experience and expertise. If there is a particularly interesting post you would like preserved from a previous forum, certainly you can re-ask the question within the context of this site and encourage folks to contribute their knowledge here. Everything here is licensed under Creative Commons for reuse, so anything posted here is designed to make the Internet a better place in general… for everyone's benefit.