Relevant versions: 9.0 and upThere is an include filter in the Scanned Content tab of the Endpoint File System Target. There is a limit of 1024 characters in the include filter, so it is recommended to use IP ranges or wildcards to group machines when possible.
From the Online Help:
Scanned Content Tab
Where you include the filters of the content that you want to scan. This includes files, file paths, IP addresses, domain names, or WINS names of specific endpoint computers that you want to scan.
Where you specify the files, file paths, IP addresses, domain names, and WINS names of endpoint computers that should not be scanned.
Note: In versions prior to V9 it was not possible to scan a subset of agents.