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: 

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?