Is there no way to link an existing GR account to facebook through Progress Through Processors?
When clicking on the link there is only a way to create a GR account through PTP.
When you click to add PTP to your FB account, this is what should happen:
(1) you should get asked for misc permissions -- this is a standard FB feature when adding an app.
(2) then you should get a PTP registration page. The first question is "Are you new to volunteer computing? (or do you have a BOINC, GridRepublic, or World Community Grid account?)" The default for this initial question is "yes". But if you click to set it to "no", then the form will change, and ask you for your BOINC/GR email and password. This will allow you to connect PTP to your already-existing GR account.
Let me know if this gets you connected -- or if the above isn't clear, or if something isn't working for you...