So I found my first yoys ,which were some fixed axle duncans.
A butterfly and two imperials.
How hard would it be to put a bearing and some response pads in them?
It seems like it would not be to bad since they just come apart, but I figure someone must have tried something like this before, so I thought I would ask here.
Even Large Size C Bearings wont fit around the fixed axle because of its thickness. To get a bearing in it, you must have to either try to shave the axle down, or possibly put a new bearing in it, but it wont be easy. I would hire a Pro Modder for this one. 
this should give u some inspiration… ;D
good luck!
I just got to see that vid and it is pretty crazy.
I think all of those options would be a little overboard.
If I can think of a way to do it that is not to expensive, I will post it.
It’s easier and cheaper to buy a decent yoyo w/a bearing.
That is of course you like to tinker w/stuff.
There’s more to it than that, you need a lathe.
I poked around the interwebz a little bit and I think I can do it with the right bolt nut, and washer set-up.
Go for it!!! I was able to get the bearing into a butterfly but I messed up the axle setup. Just never really worked on it again after that. Either way, good luck to you and I hope to see some pix of it when its done.