For logfile EventSources, it will only look at logs on which the collector is installed. The idea would be to install collectors on the servers where you want logs monitored.
If you want to monitor logs on a different device (or too many to install collectors on all of them), you'll need to move to a Groovy based ES, which could pull in the hostname using:
hostProps.get("system.hostname")
You could also pull in any properties in the same manner to use as part the path you generate to pull the files. You could use several mechanisms in Groovy to pull the content from the remote server for processing within your script (on the collector).