Surprised noone else has answered this, as I'm sure there are others here who have encountered something similar.
I have not... because I don't use IEMs or headsets with mics. Just standard wired stereo headphones. But based on a quick search of this subject, it looks like plugging a TRRS plug into a TRS jack will usually just drop the mic. So perhaps there is no need to split off the mic connection, as you seem to want to do.
If this is the case, then perhaps using a standard 3.5mm to 6.3mm stereo TRS to TRS adapter would work similarly, if you also need to adjust the size. But not really sure about any of this.
Providing a little more info on the headphones and amp you're using (e.g. brand & model) might help someone else to give a more informed reply.
Maybe there are also some instructions for this in the earphone's documentation.
