@AnaM Reading your question I'm not 100% I'll hit at what you're asking. It seems your using a non-Square email tool to distribute your mass emails and looking for ways to update your external list with new customers having joined your Square loyalty or marketing lists. First, I might suggest using Square to distribute your marketing emails and taking advantage of other resources such as loyalty, coupons, etc. I've found it very useful and no need to manage external lists. Per your question, I'm not aware of a way to do this. Possibly by setting up a custom field (Date) through Configure Profile inside the Settings option of the Customers tool you could have a record to help identify future users.
Take care of yourself and, as life provides, someone else too.
