I thought about doing this once, using household/pharmacy chemicals. The problem is that the resin is mixed, it needs to be seperate in order to refresh it as both types of the resin mix have to be treated differently. Once you do seperate the resin, which would need to be done by gravity as one type is heavier, then you would find that the seperation would not be 100% and the quality of the results would not be as good as brand new mixed resin. It is virtually impossible to seperate the resin to such a degree that you could make great resin.
The other alternative would be to by the resin not mixed, so I looked into this and could not find any. However, for the amount it costs and the risks involved messing around with chemicals its probably just as well.
The reason I could not find seperate resin I think was because its far safer, far more effective and probably cheaper in the long run to use a good RO system.