convert wall units to system cado?  

I don't have Cado units, so...
I don't have Cado units, so I can't say for certain, but I remember that the dowels just stick into the back. In which case it is not to hard to redo them. And there is a trick I learned here. You drill out the dowel with a drill bit smaller than the dowel, maybe 2/3 the size. And then you break the remaining cylinder of dowel into the hollowed out center with and awl, nail, screwdriver, or whatever.
Thus you can get the original hold back perfectly, and then clean it out with a drill bit of the correct size.
This is nearly impossible otherwise, because the drill bit veers off course.

Good advice !
For the final step, cleaning the hole, first try using the drill bit as a hand-held tool. This will avoid the drill catching in the hole and enlarging it accidentally, or (worse) grabbing, and drilling deeper than intended.

I remember reading that
someone actually replaced the broken dowels with "metal dowels" that they had cut from a rod.