There are a couple of problems with this. Firstly, changing common property to visitor parking is a change of use that requires council development approval. Strata by-laws can’t supersede superior laws like planning regulations and you could have this blocked by council.
Secondly, changes to common property require a special resolution unless they are for sustainability or accessibility improvements. Your OC might get away with a simple majority if they were creating a disabled space but that would have to be super-wide to qualify.
As a final thought, are any of the other owners using visitor parking for their own vehicles?