how to convert a slipring motor to an induction motor?

It's a heck of a lot cheaper to buy an induction motor than go out and find exactly the right squirrel cage rotor to fit your stator (it's next to impossible in fact). I suppose you could always solder windings 180 degrees apart together and then take out the brush assembly, but why in hell would you want to do that just to end up with a serious degradation in performance? Also, you will no longer have a synchronous motor, which is why you paid the high cost of having a slipring armature in the first place! That is, unless you are talking about a UNIVERSAL motor (it's not called a slipring motor)...then you have no hope of soldering windings together and expecting it to work.

It's possible but you're being pennywise and pound foolish, unless you are just doing this to satisfy curiousity.