Beste antwoord door Sam-
Welcome to the T-Mobile Community!
When you choose to take your current number to another contract, your current contract will be terminated. It is not possible to export your current phone number to another contract and use a different phone number with your current contract.
The only way to connect your current phone number to a different contract is to end your current contract (via number portation) and start a new contract with a new number.
I hope that my explanation is clear. If you have any further questions, please let us know!