To load a company route, it must first be saved using the CDU. When saved from the CDU, the route is saved in the [flight simulator]\Captain_Sim\757-III\routes\ folder, as a .rt file.

If the Simbrief flight plan is saved in the Documents\[flight simulator] Files folder, then you need to load it into the CDU, then save it, and from then on, it can be entered as a company route.

From the ‘757 Captain III’Manual Part II – Aircraft Systems (CS757CAPTAINIII_MANUAL_PART2.pdf), page 378:

A company route can be called from the navigation database by entering the route identifier (the *.rt file name with no extension). The data supplied with a company route can include origin and destination airports, departure runway, SID and STAR, and the route of flight.