Calibre does convert RTFs to ePUB (I did just that the other day), but I will warn you if you are trying to convert RTFs with any unicode (i.e. non-Latin-based alphabet) text in them. Several of my RTFs contain Korean text and those came through completely garbled in ePUB (regardless of which options I used in Calibre). So for those files, I finally just gave up on ePUB and converted them to PDF (with Acrobat which I have on my work computer). Then when I open them on the PocketEdge, I just turn on Reflow PDF so the text is actually readable.

Honestly, though, RTF is such a common text format that I was kind of surprised to see that the PocketEdge didn't already support that format out of the box (even the included Docs to Go app doesn't open RTF files).