It isn't Poser;  it is your computer's repertoire of codecs that limits what AVIs you can use.

     Any video editor, Flash compositor, or GIF compositor should be able to do a conversion.  Try re-saving the AVI (under a different name) as an uncompressed AVI.  It's a memory hog, but it totally sidesteps the codec issue.

     I have several computers, and amongst them, I have Adobe Premiere CS4, Adobe Flash CS4, SwishMax Studio, ULead/Corel GIF Animator 5 (on all of my computers!), and ULead/Corel VideoStudio 11.

     You may even have a GIF/Flash compositor without realizing it.  Some photo manipulation programs came with GIF/Flash companion apps.  PhotoShop often included ImageReady, Paint Shop Pro came with Animation Shop, and PhotoImpact came with GIF Animator 5.  You can look among your installed programs, and/or look for a launch command/button in the photo manip program.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5