Co Routes only work with routes created in the correct .rt format, so changing the extention name on a file doesn't make it the correct format, inside the file, it is still the .rte format.

You will need to create the route in the Captain Sim 767 CDU first, then save it using the CDU save option so it's saved in the CS .rt]/b] format. Once you've done that, you will always be able to load it as a Co Route.

Or. If simbrief can save in the default FS Flight Plan format, [b].PLN, then you can load it using the 767's CDU, then save it as a Co Route in the correct .rt format.

Note. PFPX (Professional Flight Planner X) is the only 3rd party program that can save in the Captain Sim .rt format that I know of.