You might get MediaInfo (http://mediainfo.sourceforge.net/en) and find out what the characteristics are of the .avi files that Poser can read - codec & settings - so you can try to duplicate them.

VirtualDub is probably a good bet. I recently found a plugin, "FLV Input Plugin"  that will allow it to read and convert .flv files. (http://moitah.net/) - you'll also need ffdshow installed (link on site). It worked fine for the couple of .flv files I've tried it on. Hopefully you can find a successful combination. Uncompressed is going to be quite large. I would think that there's a codec that Poser will work with - probably an older one I'm guessing. 

Note that the moitah.net site also has a FLV Extract utility that might help as well but you'll probably have to re-encode the extracted .avi anyway to get poser to read it, but it's an alternative if you can't get VirtualDub to read the .flv.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken