Hi Richard,
This is unfortunately not possible but definitely a great suggestion.
Could you, perhaps, make do with manually adding the user and choosing the welcome email instead of the confirmation email? This should overwrite the unconfirmed user, send them the welcome email and set them to active.
If you accept more than the users first and last name from your initial signup form this may be too much work because it means re-entering all of the users' data.
Regards