All that they can do is mimic the frequency response (and to some extent, the frequency/phase response) of the microphone that you choose. They can't make a microphone of one pattern (e.g. cardioid) sound like a microphone of a different pattern (omni, supercardioid), nor can they make a large-diaphragm capsule sound like a small-diaphragm capsule (or vice versa), nor can they make a single-diaphragm capsule sound like a dual-diaphragm capsule (or vice versa); that's just not possible.
But if you have a dry, close-up, single-mike studio recording of a vocalist singing into a U 87 or U 89 set to cardioid, they can make it sound pretty much like a C 12 or a U 47 set to cardioid might sound on a similarly dry, close-up studio recording--and that's what the technology is mostly used for.