The answer to the first question is in your second question. If the garage door is common property (and chances are it is) then you will need a special resolution, preferably one that says anyone who attaches a door opener accepts responsibility for its repair and maintenance in perpetuity. That prevents subsequent owners demanding the OC repairs their door openers when they fail years later. If the garage door isn’t common property – and it would have to state that somewhere on the strata plan – you can go for your life.