Hi skydogg.
There are a couple of ways to achieve this, the most conventional of these would be to feed an aux send to the IEMs, ensuring that all channel sends for that aux are at 'zero' and the aux is set to 'post fade' to ensure it reflects the FOH mix - then just add a little more KeyVox.
There are limitations to this:
Any non-amplified sounds which are adding to the mix (drums, bass amp, other instrument amps, wedges) which are adding to the FOH without (or only partially) going through the PA will not show up in the IEMs at the appropriate level. The listener might be tempted to stick a bit more in to balance the IEMs but knock out the FOH. You could, over time, calibrate your IEM mix to account for this with patience, wisdom and a good ear, depending on your band, venue(s), etc.
Room acoustics can make some elements appear more prominent than others, again a good mix in IEMs does not guarantee a good FOH.
I'm sure others will have experience of other issues with balance EQ, etc. It may be worth a try if your lineup and venue permit but there's nothing like being in the room to inform your decisions.