Putting a temperature sensor in the sump isn't going to give you an accurate reading - it's going to be cooler than actual.
The best place to put the sensor would be in the line going FROM the transmission TO the cooler - that way, you can get a reading on the transmission fluid after it's done its work, but it hasn't been coolect yet (giving you a more accurate picture.)
You can get away with putting a "t" fitting into the line - cut the line, splice in the run of the T, and the thread the sensor into the branch. The hose should accept a 3/8" OD tube, and you can get "T" fittings that will have a different size on the branch (stem) of the T at most hardware stores.
Check your sensor also - it may be self-grounding (one-wire,) and you'll need to make sure to ground the T fitting to a convenient chassis earth to get the sensor to read.