Hello @DemonCat ,

The flow is not based on a trigger, consequently you cannot back-populate.

Considering that you have already imported them, a possible solution could be the following:

To make the welcome stream I could recommend creating a specific flow for these contacts that has matching with a segment as a trigger.

Accordingly you create the segment that populates for example if the field: "Source" matches: "Mailchimp" or any other text.

You activate the flow, re-import the contacts with the field "Mailchimp" and since contacts match they will enter the flow.

It is not a conventional operation but I think it can work.

Ya I second @gmartu ‘s response, basically creating a Segment that includes just these imported contacts, and a Flow that is triggered when people join that segment. If you then need to backpopulate the flow, it will work just fine.