Hi Gaspard, 
You should create a flow that would be triggered by your list. You’ll then have to add the action “Update profile property”
 

Choose “Create New Property” with for example the label “Customer” and the value “True”.

Finally, you’ll have to back-populate the flow, and every profiles in this list will be tagged.

I hope this was helpful. :)

Feel free to send me a private message to schedule a call if you need help or want to optimize your Klaviyo account.

Cheers!

Hey @gaspard 

Thanks for reaching out to the Klaviyo Community for help!

@Tomb is 100% on the money!  Thanks Tomb for jumping in to help out! Adding the Update Profile Property to the list triggered flow is the quickest and easiest way to bulk update your customers properties. Also check out these other community post on the same topic:

Thank you again for being part of the community! We appreciate you both!