My impression -- it's probably more trouble and expense than it's worth, unless you reeeally really like the bike. Especially if you have to add brakes & brake cable stops & other such bits.

I've done the conversion to 3 speed on a Schwinn Breeze. Expensive and cost lots of time trying to find the right bits and make it all fit. Didn't make sense as there are so many good old Schwinn Breezes that were mfd as 3 speeds out there, and they sell for very little. Why did I do it? Nostalgia. Was a relative's childhood bike now cleaned & fixed up for her children. Misplaced nostalgia perhaps, an original 3-speed would've been fine.