Adjust-A-List is the easiest way to create and modify a new list based off an existing Twitter user's list. Simply search for a Twitter user to surface all the lists they created or are subscibed to. Then click on the list you'd like to edit and then select users you like to follow from their list, while adding additional users that you'd want on that list (up to 100). The final result will be a brand new list that will appear in your Twitter account.
Enjoy!