Changing the Expiration Date for a User

Working on a site for a client that already has a membership base that he's processing payments for manually right now. What I'd like to do is add the existing users manually, and then be able to set the expiration date to what it actually is on his spreadsheet vs. one year from add date (as this is an annual subscription).

We will be using authorize.net, but I don't want it to bill existing users now, as they've already paid until their listed anniversary date.

Is there a way to do this?