You can automate adding a flag to a contact.

Steps

Select Create in the top menu.
Select Automation from the drop-down list.
In the Criteria section, set the condition you want a contact to meet for the automation to run.
In the Automation Actions, select FLAGS
You must select two things:
1. Choose the flag that you want to add to the contacts.
2. Add a note that explains why the flag is being added. This is Optional
You can add another Flag by clicking on the Add another Flag button.
Press Save
You arrive on the Manage Automation page

Makerble User Guide