importing into a "holding pen" - subscribe later

I am populating a spreadsheet of new sign-ups for import into our supporters list. Until an email is prepared to email a special welcome message to these new supporters, I need to leave them unsubscribed so will be uploading 0's into the receive_email field. (I think that's what it's called, I'm writing this without the import instructions in front of me.) Then when the email is  ready-to-go my plan is to query on the source filename or maybe on a new tag, and subscribe all those people (without opt-in).

Here are my questions:

1. If a match is found by Salsa on a supporter's email upon import, I do understand that a new record will not be created but instead the supporter's data will be rewritten from the spreadsheet data (with some very nice controls over what to do about blanks, I must say).

QUESTION: what happens to the previously-existing supporter's groups and tags? If I specify creation of a new tag upon import, will this new tag wipe out other tags that were previously assigned to the supporter? Or is it only the info in the Supporters table that might be modified during the upload, and all related data remains intact?


2. QUESTION: Is there a better way to approach the need to upload new supporters into some sort of "holding pen" so that they do not receive blasts until the first receive a special email just for them?

thank you!

kazar