An "open sleeper" ticket can be issued for a passenger joining a sleeper passenger between any two points where the original passenger is holding the room is on the train.

I was on an AGR award ticket in a roomette on the Coast Starlight from Seattle to Los Angeles. I was able to have a friend join me betweenTacoma to Portland on a cash "open sleeper" ticket. So not going the whole way was not a problem, nor was the base sleeper reservation being an award ticket and the open sleeper ticket related to it being a regular cash ticket.

Open sleeper tickets can be a bit of a pain. They are one of the very last types of tickets that cannot be eticketed and require a paper "value" ticket. So no PDF, app display, or ticket scanning. You must have a physical ticket issued by an Amtrak agent at a staffed station, a QuikTrak kiosk, or mailed to you. Some of the phone agents at 800-USA-RAIL don't know what they are or how to make an open sleeper reservation, and require help or a supervisor to do one.