Lookalike with 1,300 Emails

I have 1,300 E-mails of clients related to very specific niche, Got their mails through other platforms such eBay etc.

If I create a website that is related to this exact niche and start lookalike audience with those 1,300 E-mails (To target similar people), would that help? Or I actually needed my pixel to “feed” it self from conversion through my website only? Thanks in advance!