AcyMailing Custom fields mapping

-- HikaShop version -- : 4.2.2
-- Joomla version -- : 3.9.15
-- PHP version -- : 7

I am not sure whether to ask in AcyMailing or Hikashop forum, but as the problem appears on the Hikashop registration page, I ask here. My client created a custom field "first name" in acyMailing (V5). She wants this to be used for registration, and now it shows up as extra field in the guest checkout - additionally to the hikashop first name field.

I would like to map them, is there any way? I guess it would be a plugin job and I researched but couldn't find any.

Thanks for any hints.