I have an existing campaign in smartlead.
Is there a way for me to create a custom field for all of the leads in it and reupload/update those prospects so that I can then use the custom field’s info in the campaign’s messaging?
Yes, that's doable with Smartlead's integrations. Just consolidate the custom variable into one column in Clay and either re-run "Add lead to Campaign" or use the "Update Lead in Campaign" instead.