I beleive Car & Driver did an editorial a few years ago and I've also seen a few other places that it is not suggested to mix Dexcool. I think there are differing opinions but the consensus I've gotten from it is to drain the dexcool out and flush the system THOROUGHLY and replace it with a compatible (to your system) coolant.
I could be wrong and suggest more research but for some reason I recall this. As for what to replace it with, THAT is the real question. I can't remember the particulars of it so be careful and maybe a few people here have done it also.
Basically I have the impression that it is a bad idea to mix it.