I am using some PBR maps from Substance Painter (Diffuse, Roughness, Metallic etc). The Principled shader has slots for these, but I how do I also use an Ambient Occlusion map? Is there some way of adding it to the Base Color of another shader an mutiplying them together? If so, how exactly?

AO is fake ambient lighting and has no place in a physical shader
if you want to use AO map as a dirt or something you can use it to mix 2 materials

How do you mix two materials?

With layer mix(if you want to mix bsdf) or color mix(in case you want to adjust only color).