-- edit --
this question piqued my curiousity enough that I ran a test for the tonebenders podcast. Check out the results here:
-- edi t--
I honestly think this is a good question that's worthy of a little thoughtfulness.
IMO it is possible to eq one mic's frequency response to closely resemble another's. As an example, I can (and have) take a simultaneous recording made with an NT5 and a Schoeps CMC6.Mk4 and use EQ (mellowing of the high end on the NT5) to make the two mics play well together and even feel very simlar frequency-response wise.
Every capsule size has inherent resonant frequencies that interact with the overall recording in complex terms though - so simple slopes and rises won't compensate for that part of a mic's character effectively.
With that said, I actually believe you can go quite a long way with EQ - especially in the upper mids and high end.
The bigger differences are in low end extension (you can't add what isn't there) and in transient response.
I think transient response specifically, is what many people refer to as "detail" and is what can truly separate the sound of one mic from another. As an example, my Line Audio CM3s have a very similar tonal characteristic to the Schoeps CMC6.Mk4 mic, but they track transients much more slowly. This means that they yield similar recordings on things like traffic ambiance, but very different sounding recordings on applause - with the Schoeps yeilding much more detail in the individual claps and the CM3 sounding very different with regads to dynamics.
The other thing about transient response is that it isn't uniform across the frequency spectrum. Its entirely possible for mic A to track transients faster at 10k than mic B (again possibly due to capsule-based resonances), but slower at 2k.
Another factor is electrical components. Mic circuit boards are filled with components that further tune the frequency output, adjust impedance, create pads and filters, and change polar patterns.
The quality of these components can do things like add harmonic distortion, boost or cut frequencies artificially, and further shape transient response at different frequencies. We recently did a shootout with a U87 versus an AT4050 on VO, and the 4050 sounded much cleaner than the U87 (though it lacked that U87 "character", probably mostly due to the differences in the level of harmonic distortion due to the different designs of the internal electronics.