You're not gonna be able to do this if you're monitored by someone. No alarmco in their right mind would accept this kind of liablilty.
If you're not monitored, just access the V15 via the installer code and add the devices. BTW and plain V15 is pretty old, are you sure that's the model number? Do you have a keypad capable of programming zones? (6139 or
6160)...if you don't you'll need to buy one of those too. The bigger issue is; do you know the installer code?
BTW if they're willing to do it for 100 bucks (and if the pirs you bought are wireless) it shouldn't take more than an hour and it would be well worth it for you to do. If they are WIRED devices...why don't you run the wire yourself and let them install them...shouldn't take more than an hour either. BUT IN EITHER CASE I DOUBT THEY WILL WARRANTEE WHAT YOU BOUGHT ON EBAY EVEN IF THEY DO THE FINAL HOOKUP.
Good luck.