A real nekomimi? Well... you can get a suit that kinda does it....just play with the catgirl in the park a bunch
Umm she should eventually tell you what to do to get a new outfit.
Ah read your first comment... and suprisingly.... I can almost see that as being possible.... It would be in the far future... but I can see how to do it. Wouldn't really break much either.... humm
I know you can push the buttons but if you get caught out or changed then most of the suit goes away.