Hi, Ihab Darwish,
Unless he were tall, he would have joined the basketball team last week.
Is it a right answer?
No, it is not.
Rewrite:
He isn't tall. He couldn't join the basketball team last week.(Unless)
Where is this question from? It is ruined. 'Unless' doesn't work here even to express an afterthought.
- He could have joined the basketball team last week– unless he is short. (Needs a very special context that is even difficult to imagine.)
It is better to use 'if' here.
- If he were tall, he would have joined the basketball team last week.