Import AD OU in SEPM for what purpose?
It is to know computers and users to know from specific OU to know if SEP is installed or not. Instaed of manually seraching for clients, you can import the OU's.
How about using GPO to install SEP clients?
you can use the SEPM to push the clients also you can use the GPO policies to install the SEP client onto clients (,memebers of domain).