No, would you like to know why?
Because you will get banned. I guess you have never heard of terms and conditions. If you knew anything about modding you would know this......
This is correct (although not with the attitude). Modding Xbox 360s and PS3s are illegal, as they could lead to a variety of issues (Ever play Halo 2 on xbox live when it came out? Real fun).
Whatever deal PS3 has with Portal 2, I'm sure they will have limited access to Steam. Mods can pose a lot of issues for consoles considering they are fan-made and can't be regulated by Sony and Microsoft.