Hello,
If you want to apply your custom field on a bunch of products ; the best is to create a special (unpublished) category outside of your general category tree.
Thanks to that, you can activate or deactivate the fields depending that category ; and via the "mass action", you can easily add category to existing products (add or remove a category for every products in specific categories).
www.hikashop.com/support/documentation/167-massaction-form.html
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.