"Pay" is the keyword! You can do anything you like as long as you "pay"!
You may want to stop by a t-mobile store and pay down the balance on iphone6, then you can start a new EIP for the iphone SE or just pay it down. They would take care of the phone number switch at the store, no need to "import" a number, you already own the number, just switching phones.
This is assuming you don't have JOD.
Good question jazmin5211.
This will depend on your credit limit for that particular line. You may be able to set up another EIP for the new phone the only piece to keep in mind is the down payment may change.