I willS tryS toS answerS yourS questionS.
Quite theological - you are right - who is a Christian?
Someone who accepts Christ as his personal saviour - easy enough.
Does it mean, then, that this person is now no longer incapable of sin? No!
So if a person stumble and succumb to sin, does this person stop being a Christian? also No!
The question is WHY is a person demon possessed?
1. Is it because he wilfully continues to dabble with the occult?
2. Is it because he has no choice but to live in an occult-influenced environment?
3. Is it because his faith his weak, and his psyche is weaker?
So to answer your original question: it depends. Scenario 1, I'd say no, no Christian should dabble with the occult after his acceptance of Christ.
Scenario 2 is quite situational.... some converts are still financially dependent on other members of his family, so he cannot move out.
Scenario 3 probably can still be considered as Christian.
Having said all that, they are just my personal opinions.
Thanks yous.