This is actually very easy to accomplish with a little PHP. You can download the script (free of course) from here at the Twitter Bot Script forum.

Keep in mind that most people only get 150 API calls per hour on their account. The only way to add people to a list through the API as of right now is to use their Twitter account ID number. So you have 1 API call to create the list, 1 API call to get the user ID automatically, and then a third API call to add them to the list. (Way to go Twitter.) This means that you can create a list with up to 74 people on it, and add people to that list at a rate of 75 an hour. Keep in mind each user is limited to creating 20 lists each with a max of 500 users. That’s room for 10,000 accounts that suggest authority and influence Muahaha.

Use this script wisely they are getting smarter at Twitter… Updated version on the way that will allow you to use either user ID’s if you have them cutting API calls to 1 request per user on the list allowing you to add 149 at creation.

I also recommend storing your created lists in a MySQL database or something as a way of tracking / managing them from your toolkit. For instance when you get the 200 OK status back from creating the list, it also returns all the xml data for it. Store that in a row so you can keep track of what you have made, and make it easy to build up.