Is it proper to propose a thesis collaboration to a professor you don't know?
A few students work with professors from another department of the same school, and in a few cases, their advisor has recently moved schools, but mostly most students are advised by professors in their own department. 

In my limited experience, professors are interested in supervising thesis projects only when they have a preexisting relationship with the student.When i was a student, I once asked a professor to advise my master's project and was told, "no, you have to take my class first".

It's more than helpful to cultivate relationships that will support your work, but when you cold-call people, avoid asking them to trust you or commit to work with you.