There are a couple of approaches that I can think of right off the top. The first would be to duplicate the keyed layer, use the top copy as a track matte for the bottom copy, apply matte choker to the bottom copy and then add a matte choker to the bottom copy to create an edge. You could then apply the stroke layer style or you could apply fill.
Another option would be to add solid to the comp below the keyed layer, use the keyed layer as a track matte for the solid, apply an inside stroke to the solid layer, pre-compose, then key out colored part of the solid.
If you want to take the time to show us a screenshot of your timeline showing modified properties of all layers involved so we can see how you set up your comp then maybe there's another easy simple solution. You can just drag the screenshot to the reply field oaths forum.