For a brief (fifteen to twenty minute) visit, probably no one would object as long as you ask permission of your sleeping car attendant first. If you're talking an hour or more, though, not very likely. If your thought is to have them share your room for the duration of their trip (which need not match yours, as long as yours is longer), then see to it that they purchase an "Open Sleeper" ticket as pennyk above suggested and have it linked to your reservation. If they do that, then they are also entitled to diner meals on board and the price should be the same as or lower than a Value coach fare (the rock-bottom Saver fare may be lower). An Open Sleeper ticket must be booked with a call to a reservations agent or directly with a station agent (in advance); it cannot be booked online. Many (maybe most) telephone agents are unfamiliar with it as it is uncommon; if you encounter an agent who professes to misunderstand you or tells you to book some other way then politely ask for a supervisor or else hang up and try again.
Please note that with an Open Sleeper ticket there is a limit to the maximum number of people who can share a sleeping accommodation. For adults in a Roomette, that number is fixed at a maximum of two. Roomettes are tiny; you're not going to be able to throw a party in there! (Bedroom Suites, on the other hand....)