Can a preexisting nvr be connected to alarm.com. either analog or iP cameras?
In short, no, an existing NVR cannot be used with ADC.
Alarm.com video service requires use of cameras with Alarm.com firmware for security and reliability. There used to be video encoders (ADC-VS121) which could allow you to use individual analog cameras, but the value is tremendously low compared to new IP Alarm.com cameras.