A splitter wouldn't help. I'm looking at one now, and it has a 3-conductor plug (called a TRS), just like a plug for stereo headphones. The plug on a mic/headphones combo is a TRRS, and has 4 conductors. I thought I had one laying around, but the plug size is 2.5mm, from an old Blackberry I had.
I'm gonna go and look in the trunk of my car. I'm sure I have one...