all I can offer is a observation from my time programming some kind of stellar system thing. Uh, don't ask about it ;)
Anyway, without thinking too much I'm afraid you need way more mass than the 400.000kg you have to offer. While the comments are right with the most important thing - everything does create gravity pull, no matter how light (except for photons maybe?) - you need to take in account that gravity pull is a force.
Like momentum. A gas giant may have a easy time to pull a NASA-Probe closer (for a swing by), but most things below... hm...
That is a guess, but to attract something with the mass of a fat cat your station might be sufficient, IF the cat isn't moving relative to the station. To attract the same thing when it passes by with some 10 km/s the influence would cause a incredible little course-distribution at best.
So use at least 10*10^20 kg if you want to catch something that moves with relative speed common in space. Or, in case of the cat, an empty box. But that's an other topic.
Two objects with the same mass or at least close to each other in terms of mass (like your station and your ship) may... I may be wrong, but to keep that ship orbiting at least a bit stable it would need an orbit that's way inside the station itself. Otherwise someone who sneezes inside would be sufficient to push it out of a "wide" orbit.
More likely is that both may orbit around a common Barycenter. And need a long time for a round. And hope there is no influenza on board.
And again, my math-fu is pretty bad. In theory you would get everything orbit you station as long as its mass isn't the same or bigger. But from my strange point of view, even a screwdriver would be able to reach escape-velocity from your station of if it is pushed away even very gently. But if there is really nothing in parsec around, it might come back one day (which could become a very bad day).
So... like I do most times, I want to ask you to provide more insight in your question-creation-process. You do have something in mind when you ask stuff like this... why you want things orbit your spacestation? But that's dealt with in comments.
I'm off for now