This is what you need: Truvativ American-to-Euro BMX Adapter

Then you can install a cartridge square-taper bottom bracket (68mm wide). Here's a link to a decent one from Shimano: SHIMANO UN55 BB Square Taper Bottom Bracket

Important note: bottom brackets come in different spindle lengths, and it's the crankset that will determine the spindle length that you will need. So buy the crankset first, and then buy the bottom bracket with the spindle length that works with that crankset.