I don't know if it would count as stacking or not.But if you're not subbed to PS now at all will a pre paid card work now because I'm not actually subbed? So technically it ain't stacking?
In the case it works, the best for you (and me, I'm in a similar case with a long PS+ sub but with no PS Now sub) would be to get a 3 months PS Now code for your country.
But they did stop selling PS Now codes many months ago and Sony disabled the option of buying PS Now on the PSN store. So it's impossible to buy them anywhere.