Once you install the beta USB drivers for the mac and reboot your computer. When connecting the EBW1150 to the mac, when you select online bookstore it will take you to your online bookstore.
If you install impserve (Download the source version, un compress the archive and run "python ./impserve.py" from the directory. It will start immpserve. On your 1150 check "Use HTTP proxy (127.0.0.1) and HTTP Proxy port 9090.
Now when you go to the "online bookstore" it will be browsing the impserve collection.
I grabbed a free book called The little white bag from:
http://www.smashwords.com/books/view/1197
I downloaded the RTF version of the book. I was able to convert this to both an EPUB and a IMP file using the "Book Designer" tools.
I then removed the IMP file and imported the EPUB file into Book designer and created a IMP from that. So to answer your question. Yes it also support's .epub conversions.
I have attached a ZIP file that shows the various conversions and has a screen shot included. (I'll attach screen separately)