So you want to replace your $8 plan with your own number?
Rightfully I don't think it's possible coz they only allow 2 lines per IC, so you'll have to terminate one line first. Then port in your ZYM line to (choose) an $8 plan.
(I can do that with M1 Maxx, but then they allow more than 2 lines. 4, IIRC.)
Unless you've $ credit in your $8 plan wallet, you can choose to terminate the existing $8 plan and port in your own number to a new $8 plan.
Service center... unless they can do something different (behind the scene), I can't imagine how that's possible...