But is it possible to place these .reg files into a folder for Addon Manager to recognise them automatically without me needing to copy and paste the key for each one?
You don't have to copy & paste. Double-click on each .REG file, and give permission to add the info in the registry. Then start the sim, and it will ask to activate, without asking for a Serial Number.
The relevant information was in the FSDT_Install_guide.pdf, at page 8:
First, let’s see the best case, when you have your .REG file at your disposal, because you made a backup of it before reformatting Windows:
- Double-click on it to restore your serial number in the registry
- Install the product Trial
- Launch Flight Simulator with an Internet connection available
- Addon Manager will retrieve your activation from Esellerate servers and, since you haven’t changed hardware, most of the time it will not consume an Activation.