ABS is usually a very integrated thing. It requires sensors, actuators, and a control computer… I have never heard of an aftermarket ABS system for either bike or car.
On a 250 cruiser, I wouldn’t worry about need ABS.
If you take the MSF course they teach you how to threshold brake. If you plan to use the bike every day, rain or shine, then ABS has merrits (though how much merrit on a 250 is up for debate). If you only ride in good weather, or on roads an hour after the rain starts (to wash away the layer of oil on the rod surface) you should never be in a position where ABS would even be an issue. If you ride in the rain a lot, then you need to give yourself more time to stop…
I guess I’m saying, take the money you would have spent improving your bike, and use it to improve the most important part of the bike (you). Then ride carefully (lots of braking distance) in situations where you are afraid your wheels might lock.
just my opinion