You cannot port a Virgin Mobile number straight to Google Voice.
I got a SIM activation kit from TMobile. Not sure if the promo is still going on, but it was $0.99 shipped, limit 2 per order.
Picked two up. Activated one on the $30 100min 5gb plan. Activated the other on the pay per minute plan.
The beauty is that the activation kit comes with $3.34 (10 minutes @ 33c/min), so you don't need to refill the account at all. Port the VM number onto here, and then port it from here to GV.
GV charges $20 for the port-in. Paid $0.99 to transfer from VM to TMO.
Hindsight, what I should have done was use the temporary TMO number (you initiate the port after activating) to sign up for another GV account.