This isn't really where the timeslots and highways are set up - this pretty much sets limits for various resources in the system - some of it is set up by what you are licensed for.
I believe you can add 2 more T1's of any kind - even though you only have 15 out of 21 PRI trunks used a PRI usually rides on a T1 and you can set up 2 more the way it is right now.
I'm not exactly sure of the engineering concepts behind the traffic management, but I know each shelf has a maximum number of traffic units and that can be broken down by half shelf and quarter shelf. Even though you might be able to say have 100 traffic units on a shelf, and even though you theoretically might be able to do it, you shouldn't configure more than 25 in any quarter shelf or you could have traffic problems. Each type of card has a certain cost in terms of traffic units - that's why you might notice that if you have a processor card on a shelf for ACD you will probably see a certain number of slots either before or after it that show as "free" - because they don't want to add any more traffic too close to that. They sometimes don't like to mix analog or digital station cards with T1 cards, etc....
Now that said, I have added a slot here and there on occasion and haven't had any problems I could easily determine.
DIMEN says the most you can have, but you have to look at BCSU to look at what slots that have (*) are configured in your system. It will show you the board the slot is configured for, and if it is not present you will have a (*) next to it and its status will be NPR. You can add boards in those slots. If you have slots that are set up for T1 boards you can probably use them to add your PRIs without difficulty. Make sure your board part number matches the board part number the slot is set up for.
If the slot is not wired out to the back of the switch you will also need an IO-Filter cable that adapts the multi-pin connector to the amphenol so you can connect your accessory cable.
I figured out a few years ago that it IS possible to change the board part numbers of the slots, and add additional boards to the database, but you must do so through the RMX port. You have to be really careful in there because you can accidentally nuke your whole switch with a typing error - and there is no backspace key in there! CHA-BCSU and ADD-BCSU are not allowed thru the normal tech logins unless they changed it in the last system release.
Hope that helps a little. If you really want to be sure you will have the traffic stuff OK you would have to ask an engineer to do a MAC quote and see where you can add the cards, but if you have T1 slots ready to go they should have already planned for the traffic.
