You are lucky that in NSW you ‘only’ need 75% to approve granting an exclusive use of common property. In the ACT one person can block something everyone else approves.
If the store room is big enough, an alternative that might work would be to set it up in such a way that any owner could use it if they wish to. Let’s say a survey could show that (say) 30% of owners might want to use the space at any particular time you could have that many spaces divided off, perhaps by just markers on floor. Perhaps you can just have an ordinary resolution at an AGM to use the space this way. You would still need a set of conditions for use, a process to review, a way for new people to come in and replace someone not really using the space. I would suggest there should not be fees for use of the common property.
If that is not practical then you should follow the formal procedure of granting exclusive use of the common property.