I was just dealing with a similar situation. Keep in mind that if you keep the xlr connector on there and use any adapter to plug into the 3.5 you'll have a heavy and awkward cable for somebody to wear (and hide) if they're moving around. I went the opposite route and purchased a mic wired for the sennheisers with a 3.5 to xlr adapter to use as a hard wired mic. It's not as straight forward as buying a simple cable. But, countryman configured it to my request.
I talked to someone at Countryman about a week ago and asked them if they re-wire mikes like this situation -- If I remember correctly I think they charge about $30 or so to do this. Another option if just in case you don't want to do this yourself.
They will do just about anything you need them to. Call and ask for Rosa - she's great to work with and makes great adapters. She made two G2-TA5 adapters for me, including a locking 3.5mm.