This may be a dumb question but no pain no gain.

I own a Sennheiser Evolution 100 series wireless set with a lav mic. I am wondering, is possible to connect a handheld mic instead of the lav to the transmitter?

This is a single shoot and I'd rather not buy Sennheiser's capsule transmitter that fits onto a mic since it costs about $300 and I only need it for a few minutes and then likely won't ever use it again. If it is not possible to buy an inexpensive adapter cable then I'll probably just hardwire the handheld into the camera.

Any feedback is appreciated - thanks in advance.

It shouldn't be a problem. You'll notice the Sennheiser's input says 'line/mic' - it will handle either level of input, so as long as you don't need phantom (or provide it another way) it should be fine. Just get a short XLR-3.5mm cable.

The only reservation I have is that on the older ew100s, I've had issues with putting a non-locking 3.5mm plug into the transmitter. I don't know Sennheiser's wiring scheme and maybe I need to wire it differently. When the plug was all the way inserted into the receiver, I got a trebly sound with no bass. When I move it up a notch (so the tip is probably hitting the ring of the jack), it's fine, just not a stable connection.

Sennheiser CM-1

XLR Mic level connector on one end, locking 3.5mm plug on the other end.

For line level sources, use the CL-2. Same cable as CM-1 but wired to use the line input of the transmitter.