You can. Koodo doesnt care what you do with the phones after you have them.
If you are buying the iPhone 7+ from Koodo you will have to pay the tab off first.
If you are getting the iPhone 7+ somewhere else, you can simply put the SIM into it. You will stay pay your tab amounts on the iPhone SE though.
You can get the iPhone 7 you will have a tab that will be added to your next bill. So if the iPhone 7 is free or it cost 100 dollars you have to pay that upfront and the tab will be on your next bill plus any left over charges from your current plan to your billing date.