Yes, it is very simple.
We took an existing ATT landline number and at an ATT store, purchased a SIM card for ATT Gophone at a cost of 11.00
That very same day, converted the ATT Gophone, which is a Cell number at this point, to a Google Voice number.
Then simply signed up on Project FI
This same method works also for converting any old landline number to a Google voice number (without using Project Fi) and having a free housephone on Google Voice with free USA long distance and free fax service. Your existing phone jacks even work at house since you can connect Google Voice to an OBIHai device available from Amazon for around 40 but no more monthly phone bills.