Circuits vary, so there's no one-size-fits-all circuit hack. You may want to collect mics for which there are known good hacks, like the BM-800 type, AKG C3000B, AKG Perception 200/220/400/420 and P220/P420, Rode NT3, etc. (And capsule swaps for several models of MXL mics with Schoeps-type circuits.)
If you're willing to use an EQ plugin, or tell other people to, life is easier. There are many relatively inexpensive large-diameter condenser mics which use a copy of the Neumann K67 (or K87) capsule, but fail to EQ it the way Neumann's mic circuits do. These capsules generally have a pronunced bump in the frequency response at around 10 KHz, and you can use a broad gentle notch filter centered there to tone it down by a few dB. (Or a shelving filter with a corner frequency a bit lower than that, to roll off the bump and everything above it too, like Neumann's circuits. That's probably fine for a vocal mic.)
A lot of inexpensive MXL mics use a Schoeps circuit with flat frequency response, but pair it with a bright capsule, so they're bright. You can swap in a capsule with a flatter frequency response, better suited to the flat circuit.
To know what to do with a given mic you may want to learn how to run sinewave sweeps with REW and measure that mic.
Short of that, you may find useful information about various mics at AudioTestKitchen and at the RecordingHacks Microphone Database. ATK has information about current models of many mics, including frequency response plots that are more honest and realistic than the idealized plots that accompany many mics' published specs.
The Microphone Database can tell you things like whether a mic uses a K67 type capsule, and for an old mic, whether there's a more recent version that you may find information about at AudioTestKitchen, which probably sounds about the same and can be EQ'd similarly.
