Hi @neharathour
Thank you for posting in the Community.
Unfortunately the leads will not be backfilled, but you can easily export these from Meta and then upload them to your list in Klaviyo.
In order to setup a flow for your leads, I would recommend that you setup two flows - one witht the filled out leadad metric and one for your uploaded leads, ensuring they get a good start in your list. Remember to filter these out from your general welcome flow, so they get the right message :-)
I hope this helps!