How load joomla registration custom fields in hikashop registration form?

Hi,

There is no option to do that.
Supposing that this component stores the phone number in an extra column of the users table of joomla, what you can do is to create a custom field of the table "user" via the menu Display>Custom fields of HikaShop to have a similar field on the HikaShop registration form.
Then, you can configure a mass action via the menu System>Mass actions.
In it, you can have a trigger "after a user is created", and an action "update the values" and select the phone number column, the type "operation" and the value user.xxx where xxx is the column name of your HikaShop custom field.
That way, the mass action will automatically copy the phone number from the HikaShop custom field to the phone number column in the user table when the user is created during the registration process.
If that component doesn't store that in the joomla users table, then it will require development of a small plugin of the group "hikashop" to replace the mass action in order to store the data where needed based on how that component is developed.