For EN 446, no
It's a night train and they do not want people to get on in Potsdam. I asked them on Twitter, and they responded. (I had a typo in the train-number.)
Here's a free translation to English:
Me: Why can't I book EC446 from Cologne to Frankfurt/Oder online although it stops there?
DB: Do you mean EN 446? In Potsdam we don't plan for people to get on, only off. That's why this station cannot be booked.
Me: Yes, that one. Can I book from Frankfurt and get on in Potsdam anyway?
DB: No, because in Potsdam only deboardings are planned, not boardings ;-)
Me: Are there bouncers? Can I get on later in regular ICE trains? For example if I have a ticket from Berlin to Cologne, can I get on in Spandau or Wolfsburg?
DB: Our people are on the platform. Of course you can get on later, but remember that seat reservations are forfeit after 15 minutes.
So we can conclude that on this particular train, you cannot do that. We don't know why exactly, but if you try anyway and they catch you, they might not let you in.
In general, yes
You can always leave early or get on late in regular trains. Your tickets with Deutsche Bahn give you the right to be on the train (or any train on a direct route) between two stations. They frequently do not check tickets between two or more given stations.
There is no way for the conductor to tell if you sat there for one station, two stations or longer if they are passing through the carriage for the first time to check tickets after the fourth station.
It can also happen that you get never checked because you got on at the other end of the train and get off at the next station while they are still busy somewhere else.
The above Twitter conversation confirms this personal experience.
Bonus-Information: If that happens and your printed ticket does not get checked/clipped your City-Ticket in the destination might not be valid. You need to go get it stamped at the information desk in the destination station or local public transport might fine you for not having a ticket.