You need to extend the correct contextmenu. The following link describes in words (no source code) how this can be achieved:
Shared Addin using Word
Maybe this Link might help a little with the coding. I haven't tried it out myself, but it might point into the right direction.
Good luck! :)
Edit:
Does it have to be the ribbon style context menu or would a button within the normal context menu be enough?
In case the normal menu would be ok, you might use this way (C#):
Microsoft.Office.Core.CommandBar cb = this.Application.CommandBars["Text"];
Office.CommandBarControl newButton = cb.Controls.Add(Office.MsoControlType.msoControlButton, missing, missing, missing, missing);
newButton.Caption = "Test";
newButton.Visible = true;
newButton.Enabled = true;
You can do this with VSTO, I'm not so sure if it works exactly the same way with the shared Add-In technology, but maybe it does help ;)