Yes, this is possible in 0.4 - added cars are fully supported.
They're managed by LSPDFR, so they will have the ability to arrest - we'll have more to say about police AI sort of stuff soon.
This is a good point about setting the response time. It's not something that is currently possible, but we'll definitely look into it. One of the cool things about 0.4 though is that as you call more units, they do arrive from further and further away. This can be quite interesting if you're close to the border with another agency's areas, as eventually the other agency will start to respond too which makes for some nice looking scenes especially in Davis if you have it set up to be patrolled by the sheriffs.