She will never be an official participant. Leora agrees that Zena is not a participant because in this way she can basically have no "obligations" to do shows like with Malia and above all so she can't have her friend around when she wants to be alone. A true coexistence would end up in the same situation that was created with Malia, but as it is now Zena can do in and out of the apartment quietly, have her spaces without the pressure of the cameras, and Leora would have her spaces when she wants to be on her own.
Another problem is that Zena has already an apartment to live in, even though she lives with other girls. If she goes to live with Leora and leaves the other apartment, she may not be able to return later if she leaves RLC. It would end up living in a forced way with Leora and it is not certain that coexistence is always good (especially if forced).