Anyone have thoughts on how to add one or two additional deep cycle batteries to power our stereo upgrade. We are adding about 3000 watts of amps..... When do we need to upgrade alternators to HO or can the existing alternators be upgraded somehow to avoid buying new ones?

How to add batteries depends on what you have now. It can be done using Perko switches or battery isolators or both. With 3000rms watts, you should probably have at least 2 dedicated stereo batteries.