I haven't tried writing a converter, but you can get the relevant formats at Wotsit (I absolutely love that web site). If you write the appropriate RSR header out in place of the PICT header, I expect that Poser won't have any trouble with it.

You can get the header and other relevant image file changes here.

 There's a link here for writing an RSR file header in Perl, which you might possibly find useful.