Yes you can - I wrote a blog on this - Troubleshooting Sitecore Marketing Automation Plan Enrollment - relevant excerpt below:
How can I trigger contacts to be processed by the Automation Engine?
Basically — they need to be submitted to xConnect somehow. A few options here:
You could “touch” them somehow — for example if you’re syncing via Data Exchange ...
If you have a look at the release notes for 9.1 Initial, it calls out "Marketing Automation now supports seeding a plan from a contact list. All the contacts from the list are enrolled in the plan."
The wording ...