I have integrated Nmap into NetScan and it does detect the OS on some of my devices. However I cannot put this particular device on our domain because we are also using this device to do scans for auditing purposes for our customers. Most of our customers are in a domain environment. Do you have any advice on how I should proceed?
It's not very clear what you meant. So here are two answers:
If you use the device at customer's site and you are given access to their domain and an admin account, the the OS detection from the Workstation tab should work. Alternatively you could use WMI, see Options - Remote WMI - Windows name.
If you are not given any access credentials, then Nmap would be the only option. It uses advanced TCP/IP stack fingerprinting for OS detection that allows you to discover the systems without access credentials.