Options in Characteristics

-- url of the page with the problem -- : mywebsite.com/mypage
-- HikaShop version -- : x.x.x
-- Joomla version -- : x.x
-- PHP version -- : x.x.x
-- Browser(s) name and version -- : XXXXX x.x.x
-- Error-message(debug-mod must be tuned on) -- : Error_message

Hi. i want to know how to make an extra options in Characteristics
for example i am shoes in my website and i added Characteristics "Size" and mentions different sizes in it like 40, 41, 42, 43, 44, 45, custom.
i want if someone select custom in Characteristics options it places a row below the Characteristics options to type the customized size the user wants.. how is it possible ??

here is the origional one