Create a list of IP addresses (plain .txt file, each IP on a single line) for the systems in question.
Place the file in a folder on the ePO server.
Create an ePO Task (which can be scheduled to occur middle of the night) to import that file as systems, and deploy McAfee Agent to those machines.
If you want to be fancy, you can even create a new tag, such as (new_client), and have that task (above) assign the tag to the imported clients. Then, in the morning, all you have to do is review your system tree for that tag and determine which ones did not 'accept' the agent.
Rinse and repeat.
In the meantime, if you are synchronizing from a domain, all domain machines should show up in the system tree. If they do not, check your LDAP synch task to ensure it is running correctly.