Alright, so you want a separate script to send an email to a group of imported users. This would be a fairly involved add-on. The following would need to be done:
1) Import users into a new list via uploaded file
2) Insert the message into "Sent Messages" so it can be queued
3) Start a new queue/batch ID and insert entries into the queue table
4) Start sending (call to ListMail's domail() function with the queue batch ID)
If this is all correct I estimate this would take me about 3-6 hours work to develop an interface and get right. I would have to charge $200-$250 for this and, additionally, do not have that kind of time right now.

Perhaps there is an easier method. We could take out some steps, such as the complicated queuing and sending, by using the existing ListMail scheduler. It's fairly easy to insert a message into that process, but I understand you may not want your messages to be sent at the same time every day. To solve this, dailymail could be run more often than once per day but with special options to skip certain procedures, such as followup sending.
That would leave us with the more straight-forward importing of users and scheduling of messages (1-3 hours).
Hopefully we can figure something out that will work for you.
Regards