A normal steroid cream shouldn't affect your immune system like that. You're probably taking something like Halobetasol propionate, which can affect your skin as it's a strong steroid. Your immunal response might stem from the psoriasis but the cream shouldn't be exacerbating it. The only drug off the top of my head that's been used for psoriasis use that can affect your immune system (and kidneys too) is methotrexate. It's a cancer drug that was found to be very effective for psoriasis. It's pretty potent though and you won't usually take it unless your condition reaches above a certain percentage of your body and even then you won't take it for very long, maybe 6 months. I took it and it's not fun, trust me. While I'm not a doctor from what I've been told psoriasis is more akin to the way cancer works. It also can lead to psoriatic arthritis, which is very painful. I keep mine under control fairly well.
Anyway if there could be anything gained from the way psoriasis works to help the healing process I'm sure they've already explored it but for all we know about it psoriasis is still a pretty big mystery and we're really no closer to a cure than we were when I had my first outbreak at 2 years old (I'm 34 now).