You can use the SIM card you already have for a new application. 

Navigate to "Add SIM" in the customer account.

You can find a step-by-step guide here:

1. Navigate to "Add SIM":

2. Select "Activate SIM card":

3. Enter the ICCID of your SIM card. 

4. please note that you can only request porting if you activate the voice option. (Voice

option with "Yes") in this step.

More detail can be found here: How can I take my existing phone number with me?