1) let us clarify - logs described by the given link are actually already "logs from agents". So you can just download the fetched logs to the computer into a directory and add this directory as a source folder for the job. Agent must be installed on the computer where you download fetched logs. Another option is to add /data/client-logs to the job directly. Use the Management Server agent as source agent.
2) If you actually seek to backup MC's logs - which are common.log in MC's storage folder - you can use a pre-indexing script and copy the common* files out of storage into some directory and use this dir as source for the job