What program are you trying to import this AVI into?  As the others have said it is best to render as single frames and assemble in a Video Editor, mine is Magix.  However if you want to render as AVI change the background to Green or Blue or some other color that your program will be able to use as Chroma Key (green screen).  Load and select your chroma key and new background.  Not the best since you will end up with little bits of artifacts around the edges.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13