Adding Coolant to Dex-Cool

I need to add some coolant to my 04 Malibu. Several of the Coolants that I have read say they can be added to any Coolant in any car. I am a little leary of this (not sure why). Are these products ok to add to the Dex-Cool that is in my car. None of them specifically said they could be added Dex-Cool. Thanks for your help!

Jason

I would just add dex-cool to dex-cool

Don't mix your coolants. The original coolant is good stuff.