could i put a 3 piece crank with closed bearings into a bike made for a 1 piece crank with loose bearings?

I was going to do this same thing before I ended up just getting a new frame. I found this on sheldon brown but it is cheaper to on amazon.

http://www.amazon.com/Truvativ-00-6415-027-000-Ame...

Here is the link on sheldon brown

http://sheldonbrown.com/harris/opc.html

I reccomend just replacing it with a loose bearing 3 piece as it would be cheaper, I have done this same procedure on a second hand bmx, rode fine on the one piece for a while then bam, it came loose and totaled the bearings, swapped the bearings and the one piece for a 3 and now it runs smooth as spider silk, just remember, clean the bearings before installing and regrease them and service regularly as you would with a car or motorcycle to keep it running, you may also notice a slightly better power delivery too when you install the 3 piece

yes u can convert and unsealed bottom bracket and 1 piece to a 3 piece cartridge set up, but u will have to remove the bearing races and take it to a shop to get measured for the right bearing and bracket kit

http://sheldonbrown.com/opc.html I think it may be possible but not worth the effort.If your bike isn't running properly replace the chain first that will probably resolve your problem.Changing the bearings from loose to a cartridge requires delicate measurement,a bike shop may do it but it's not something to DIY.

Here's a discussion on how to do that http://motorbicycling.com/showthread.php?t=11144

It may be worth you while to look for a used bike rather than trying to repair yours