If you are a registered degree student at a Finnish university, you are entitled to apply for minor subject studies at any other Finnish university. This is called the JOO agreement. The studies should be of a nature which you would not be able to undertake in your home university and which can be included in your degree. Both bachelor's, master's and doctoral students can apply for the JOO study right.
You can check the application times for each university on the JOO-studies website of the university in question.
JOO studies are free of charge for students, but universities charge each other, and the student's home university pays the study fee. For this reason, when planning JOO studies, you must first obtain the endorsement from your own university for the studies you want to study.