Hi,
The customer can ask to be an affiliate by checking a checkbox in his account page.
The plugin allow a custom fee system, you can change/override the value for a specific user in the backend ; you just have to edit the customer (or partner) and select "yes" for "Custom fees".
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.