You'll probably be best off building a table and attaching it to the rails that extend past the iron top and support the fence. You'll probably have to drill some holes in the rails. Getting the router top even with the top of the iron may be a challenge. You might also consider the alternative of making an outfeed table with the router underneath. Rockler has a kit for mounting the outfeed table. My outfeed table is made with 1x trusses underneath to keep it flat. Use some very flat Baltic Birch ply that you hand select for flatness. You might want to add a hardwood edge to screw into through the rails.
If you do a double layer, 1/2 inch on top, 3/4 underneath, it is easy to cut the opening in the top to fit the router mounting plate, and cut the hole beneath smaller to leave a lip. Then you can use screws or the Kreg leveling device to the height just right. If you're interested, you can overlay some laminate on top with contact cement using dowels to align the laminate, then remove them one by one and roll the laminate down with a J roller. Start from the middle, and work toward the sides to avoid trapping air. Read the glue instructions carefully.
I have a separate table, which sits near the front of the saw so I can pull it over and use it for infeed support. The one issue I'd have is that when using the extension for the router, you can't use the saw. Not a huge thing, but once in awhile the issue will turn up.
