That's a great question and one that we certainly hear a lot. Currently, the only way a contact can be added to an automated workflow is if they meet the requirements of whatever trigger you've setup in that workflow.
The available trigger options are: Signup: Triggers a workflow when someone signs up from a form or integration Date-based: Triggers based on a date stored in a contact's record: birthday, anniversary, appointment, etc. Field change: Triggers when the data within a field in a contact's record is updated, like if their Purchase Total field changes from $50 to $100 and you want to target them differently Link click: Triggers when someone clicks a link in a mailing you've sent or scheduled
With this info in mind, I know that a common method used to add many contacts into a workflow at once is to take advantage of a date-based trigger. If you create a date field for the purpose of triggering automation, build a workflow that is built to trigger off of that field, and then re-import your contacts with the date you want the automation to start, all of the contacts with the newly imported date would successfully trigger the workflow.
If you aren't interested in going that route, you could also choose to schedule a series of mailings that send to any of your groups.
I hope this information is helpful! Let me know if you have any questions on this.