Missing variant option for between sizes

Hello,

We would like to add products that have in between sizes such as: 16/17, 18/19, 20/21, 22/23, 24/25, 26/27, 28/29 and 30/31, but if we are adding it manually there is no option to use that, just sizes suchas :16, 16.5, 16 1/3, etc...

We saw some of the sellers have those in between sizes on the listing and we've searched everywhere and can't seems to find an answer how to achieve that? Any idea?