This was actually covered in a thread I started on August 23rd entitled
Stupid Question or Brilliant Idea? https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/111899-stupid-...ml?src=subsct2
I pretty much got torn to shreds for even suggesting such a CRAZY idea (though I still think it's possible). Looks like this thread is a little more polite.
Don't get a Raynox HD-6600pro. I purchased one to do exactly what I wanted to do and what you are proposing. It sux...... royally. Extremely soft about 1/2 way out from center. Basically, if you don't mind the center of your image being marginally sharp and the rest being completely soft and even blurry in places, then by all means get one. I'm sending mine back to the guy who sold it to me. By the way, Raynox advises against using the DCR or HD6600pro on DSLR's. I contacted them after I used it for the first time to see if they had any input. Horrible customer service by the way. They are very rude. I will never, and I mean never buy another Raynox Product after the way I was treated. I've been coveting a DCR-150, but now I don't think I want one anymore.
If you want a wide angle conversion lens that is pretty good, just a little soft at the edges, then get an Olympus WCON-08. Only 0.8x magnification, but it's better then nothing. I own one personally and use it all the time when doing interior shots of homes. At least until I can afford a Sigma 10-20mm.
FYI to all those that have mockingly said "get a film camera" I say I SHOULDN'T HAVE TO! Either come up with something that makes my full frame lenses 'act' like their focal length says they should 'act' OR Pentax needs to come out with a full frame DSLR. That's it, end of story.