Can I write in my SOP that I want to join labs which aren't belonged to the department I apply?
This entirely depends on the university culture. Some universities and labs are very free in terms of the flow of faculty and students, others are much more siloed. It's impossible for us to give you a general answer. 

You should instead talk to students and postdocs enrolled at that university and in those labs and ask them whether it's possible. They are the ones who are most likely able to tell you what the situation is there.