First getting your Republic number to Google Voice is possible but not straightforward. This is what Google means by use your existing number. More on that here: http://republic-wireless.wikia.com/wiki/Transferring_a_Republic_Number_to_a_Service_Provider_That_Refuses_Landlines.
Next, as to whether you’re able to keep your current plan with a new Republic number (required if you intend to continue Republic service), we’ll need to know more about that plan. Please open the Republic app on your phone and quote, for us, the words underneath your phone number. If upon opening the Republic app, you don’t see your phone number, please swipe left until you see the “Your Phone Info” screen.
Finally, if you’re receiving a lot of spam calls, I suggest that you reach out to Republic support and open a ticket to see what they might be able to do to help.