I just answered this for you in a visitor's message, but it is a good question and I will again answer it here.
Answer for all:
Yes, I know of (and created) the software to convert .prc to .rb; it's called Mobi2IMP and can be found in the thread Mobi2IMP 9.4 with new Windows GUI & UTF-8
You will note that there is an option to convert the .prc/.mobi/.pdb file directly to REB1100 .rb along with the .imp for the EBW1150, namely:
For .LIT to .rb, follow the instructions (again created by me) to convert .lit to .imp as per the thread Convert .lit directly into .IMP formats
Then ensure you have created that empty file 'debug' so that the .html and images are left behind. Open the o_sb.opf with eBook Publisher and choose as the Build Target 'Monochrome Half-VGA'.