Most good alarm companies wont need the engineers code. It is a lazy way to take on a system, the new company should be defaulting the system and reprogramming it from scratch and a Veritas is not that big a system so should not take 20 mins to default and re-program. Defaulting and reprogramming is the only way to ensure that the system they are taking on is programed as it should be, and it is their own interest to do this as if it fails to perform they could be liable
Alternately you can ask ADT to default just the engineers code, but they are entitled to charge for their time