It's important to remember why you're going to university in the first place -- to study for a degree in a specific subject area. Some courses give the ability to do units outside of that subject area, to widen your experience. Some find this a positive experience (they get to do something they're enthusiastic about, and do well in it); others find it a negative (they aren't really enthusiastic about the subject, and don't give the unit the attention it requires, and do badly in it.)
Some course simply don't have the time to fit in what they require you to learn while allowing you to do an open unit. That means that open units aren't an option. That's an indication that they don't think there's time to commit to the course that you're studying as well as something else.
I don't know if there's a formal way to attend lectures outside of your course. I don't know if it's technically permitted at all. (Bear in mind that your tuition fees cover the cost of the lectures you are attending as part of your course of study -- not to attend whatever other lectures you fancy wandering along to.) But -- if only as a matter of common courtesy -- you should ask the lecturer's permission before doing so.
To be honest, you're asking about philosophy lectures, and now law -- which makes me wonder whether you have the right level of commitment to the course you've got a place to study. The last thing you want is to find yourself distracted from things you're supposed to be doing by your other interests.