Users are not added to segments. A segment is just a bunch of filters that will be applied when you delect a segment on an action ypu want to perform.
There's no need to use an extra list or Automation for what you want to accomplish.
If you want to send a series of followup mails to new users on your main list based on the way they subscribed you can set it up like this:
To start with, Acymailing stores the source of the subscriber, wether this is an import, module or form.
First thing you do is to setup a segment for each source you want to send a series of follow up mails to.
The nex thing you do is to is to setup your series of follow up mails. Set the trigger to when the users subscribes and set the conditions to both your list and your segment.
After that you can write your follow up mails and your good to go.