If the spirit possesses you, he controls what you do. What you want requires an initiation to get the Channeling power (SG, p148). The spirit cannot use your powers/skills when possessing you, only what he, himself, bring to the table, except that (if I remember right), his physical stats become yours + his Force / 2.
In any case, the spirit is an astral being, so even when possessing something or someone, it is present in the astral (with the dual nature in that particular case).