I suspect that the 'existing' patio is in disrepair. Assuming that, it would not be prudent to do as you plan.
If the concrete is cracked and separating now, chances are, that after you poured your new one on top of the one you have, the new patio would do the same as the old one. In short, it would be best to re-do the base to get a better pour so your new patio doesn't end up like the old one.
If the existing patio was NOT cracked and/or NOT in disrepair, then one would ask why you would want a new one. I hope you understand my explanation. My best to ya and hope this helps.