I don't see why not, as long as the factory insulation is going OVER the hushmat or dynamat. It's not super thick stuff and I can't imagine it being a show stopper if you're going to put the insulation down and then carpet.

 

The only issue would be that the adhesion of the Hushmat or other adhesive-backed sound deadening material depends on harmonic cancellation in the metal by damping the sound from the frequencies created by the exterior noises amplified through the metal by adhering to the metal itself, creating a damping layer by absorption as well as the cancellation. I'm sure using it on top of the original insulation would work a bit, it just may not be as effective as it is listed to be for db reduction. you will have to copmpletely clean the interior surface of ALL debris and old insulation prior to installing or it won't be as effective.

 

I replaced the old jute padding attached to my '69 rubber mats with new similar thickness padding. I put that over dynamat which covered most but not quite 100%
of the floor. The mats fit fine, screw /bolt holes etc. lined up no problem. Dynamat is about 1/8" thick. The floor pan had been replaced so it stuck really good.
Any sound deadener of around the same thickness should work fine.