How to import Papers references in Authorea
First download the Authorea style file, and copy it to the Style directory of your Papers library. If you don't know where your Papers library is, you can find its location in Papers' Preferences, under the General tab.
In Papers' Preferences, choose Citations and select More styles in the Favorite Styles menu. Look for the Authorea style, and check the box in front of it. You can now select Authorea in the Favorite Styles menu.