It's not that simple. The transmission is sending all kinds of data to the PCM. Input speed, output speed, detected slippage, which gear is commanded, which gear is engaged, lockup commanded, lockup engaged, variable line pressure, prime state, oil temperature, etc., all of which are used to calculate sets of expected data (PIDS). Sticking a resistor on is not going to cut it. Very few of these data points are read as a static voltage, rather as a variable frequency signal that is reduced to numeric values and related to data tables in the PCM. Assuming you were able to somehow fool the PCM into turning off the CEL, it would still be missing expected data, and would trip the limp-in mode flag after a certain distance, which would turn the CEL on again.
The solution would be to find a PCM from the same model truck that had that transmission installed (if it ever was), or alternatively, flash a PCM with the correct firmware, which would be essentially a wildcat operation, as no dealer will flash a PCM with non-stock firmware. The swap itself is technically illegal from an emission standpoint, as it's not a certified combination anymore.