Hi,
You can't and I don't think that the copy feature is even possible because a custom field create a column in the database ; and you can't rename that column afterwards.
But if you want to duplicate the fields to plate them to different categories, the best is to select multiple categories on the "unique" custom field.
Because HikaShop (2.5.0 for sure) handle multi-categories on product custom fields.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.