I am going to put aftermarket rims on my 2005 Cadillac STS. I won't have the TPMS sensors initially... Is there a way to disable the TPMS system (and avoid the low pressure warning or other TPMS errors) until I have the TPMS sensors installed in the aftermarket wheels?

That's a NO.
What wheels are you going to?

i've not seen this done on the STS, but the CTS-V guys found a way. search that forum and the FAQ www.cadillacfaq.com
also, why not have the TPMs installed in the new wheels? dont fit? have the shop band them to the wheel.

I believe the sensors are a part of the valve stems. I had them installed on aftermarket rims without a problem. Just have to reprogram the car for the new set of sensors. Instructions for this are in the owner's manual. I think the sensors were around $20 each.