Prosthelytize?

Is the "prosthelytize" an accepted variation of the word proselytize (or proselytise, in British English)?

I don't think so.  It's probably more than a typo, but not in common enough usage to be an accepted variation.  My guess is that many people are unwittingly substituting the familiar (and easier to pronounce) "prosth-" prefix (as in "prosthetic" or "prosthesis"), not realizing it denotes "adding on", as opposed to anything having to do with encouraging religious conversion.

Resources:
http://linguaphiles.livejournal.com/5197417.html
http://www.etymonline.com/index.php?term=proselyte&allowed_in_frame=0
http://www.etymonline.com/index.php?allowed_in_frame=0&search=prosthetic&searchmode=none
http://en.allexperts.com/q/Etymology-Meaning-Words-1474/2010/4/Spelling.htm