CanBeBaseForCharacter is a virtual function defined on the Actor base class that returns true if the character can walk on it. It’s not exposed to Blueprint, but that’s not really a problem since it defaults to true. That means it’ll be true for every actor unless you specifically override it. CanCharacterStepUpOn is the equivalent for components. If it’s true, the character can walk on it.
Will your space ships move freely in 3D space and will characters be inside during that time? in that case you need to take some care to stop the character movement component doing its thing. Also, if you plan on having AI characters, do note that Unreal Engine’s navmesh implementation only supports Z-up.
Still, with the little I know about you overall project goals it seems to me that my suggestion above is the easiest and most straight forward solution. Are you still gathering ideas or have you tried anything?