Unfortunately at this point there is only one welcome email, which will be delivered to anyone that joins the list using the "ACTION_BY_CONTACT" opt in source. You can find more information about this in Opting-in A Contact.
The only alternative that I can offer at this time would be to set certain subscribers up on an autoresponder list. This would allow them to recieve a series of emails set up at specific intervals. Unfortunately there is only one set of autoresponder series that can be sent out. For some more information regarding autoresponders, I would recommend viewing this FAQ.
I apologize for any inconvenience that this issue has caused. Please let us know if you have any other questions.
