adding client to 800xa domain
If I were you, I'd clone one of the existing clients, change IP-address and hostname and finally join it to the domain and later the 800xA system.
Cloned computer SIDs is in general not a problem for System 800xA; but if you like you can have the cloning tool to spawn a new SID for the cloned client.
Otherwise you can create a new setup package or reuse one of the old setup packages (but change IP address/node name) and have a go at all the vast amount of DVD/CDs, setup.exe:s and man-hours that need to be overcome before a new client is born.
During your system planner step, how many clients did you set? If you've set more than 5, you can use this system planner package.
If you don't have more then 5 OS/ES, then you have to install predefined node as an AC 800M Engineering Client, pay attention please do not install as a AC 800M Single Node Engineering System.
After installation, you can add new client to your system.
Hope it helps you.