wh开头的从句可否放在介词之后

可以。
例如this is the house in which i used to live .

好象是不可以的

可以，比如说介词+关系代词结构。

He is the boy with whom I was just taking.

可以的,在重句中常常使用介词+WHICH 代替WHERE
当然也有别的..
EG:I MET THE FRUIT-PICKERS,SEVERAL OF WHOM WERE UNIVERSITY STUDENTS.
THE ENGLISH PLAY IN WHICH WE ACTED AT THE NEW YEAR'S PARTY WAS A GREAT SUCCESS