The Supramental Avatar simply means that the Supreme Divine deliberately and consciously assumes a human body with the purpose of opening the possibility of Supramental evolution. Before this happens there can be no evolved supramental being as there will be no individualized supramental body or kosha. An evolved psychic being could at most evolve upto the overmental being before the Advent of the Avatar who comes to open the doors of the Supermind.
Now that the doors have opened there will be supramental beings created through a process of evolution just as subsequent to Sri Krishna’s advent there have been devotees totally identified with some aspect or the other of Sri Krishna. But the Avatar, the pioneer has to be so much more equipped for the work armed with the very highest powers for the purpose for which he came. The psychic being evolving into the supramental being will be in essence the same but still there would be a difference. It will be representing only those modes of the supramental Truth that it is meant to embody.
Is it, The psychic being of a Supramental being is connected to supramantal plane(maharloka) but the psychic being of Supramental Avatar is connected to para raddha plane(combined of Jana lok, Anand lok ,Satyalok, means that plane of pararaddha)?
Well that is one way to put it.
Supra mental body is the final evolution of human being and then after when human completed the praraddha of super mental being then the being will get the bhava deha whose psychic will merge into higher plane of pararaddha like Sri Aurobindo 
You told me Avatar has his own psychic being like Sri Krishna. So when Jeeva get entered into Leela with bhava deha he also gets the psychic of that plane like bhava deha …?
Bhava deha is not the evolutionary body created through the action of the supramental force upon matter. It is built in the inner being or more specifically that part of the inner being which is consecrated to the Divine, especially for a bhakta. The soul then can, after departing from the physical body, dwell in the higher realms with its Ishta Devata, for example Narad. But that is not transformation which refers to the very physical body and earthly life getting transformed by the supramental consciousness and force. It is not merely the psychic being dwelling in the supramental plane or upon earth while in contact with the Supramental forces. It is the mental and vital bodies and eventually even the physical body getting transformed and hence escaping the law of disintegration and death.
So, when people get entered into Sri Aurobindo’s loka they should also have a body or psychic like that plane?
Sri Aurobindo loka is in the subtle physical world because the work of physical transformation is still going on and is not yet complete. Those psychic beings that are developed enough to be in a living contact with Sri Aurobindo remain there in the subtle bodies and various kinds of works are going on in them. When the world is ready and the time comes the beings of this realm will enter upon earth to complete the evolutionary process.
Affectionately,
Alok Da
