When you enter a new ACE in a named ACL and include a sequence number the switch inserts the ACE according to the position of the sequence number in the current list of ACEs. Let SQL Server Do It. Lori Hyde shows you an example of how Ciscos introduction of sequence numbers makes it easier to manage Access Control Lists.
To insert a line in between entries you just need to find a number in between then add the line starting with that number.
So if we have lines 10 and 20 we can use 15 or 11 or 17 you get the idea and just add it directly. You can then insert or delete lines by the line number. If the user enters an entry that matches an already existing entry except for the sequence number then no changes are made. Idea is create an another list say Counter with a field to hold your initial counter value.