Store a category in a product custom field

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.6

I need to create a custom field where categories would be stored the same way the core fields do for storing the categories of a product.

i mean that when my custom field gets focus in a form a category tree shows and I can choose at least one category. Those categories won't be used as to determine in which category the product is classified but for other purposes of my project.

Is there a simple way to do this, or not simple, but that you could guide me as what would be the right code to put in my customized data entry form ?

I may be wrong but I don't see the existence of such a type of field in the custom fields construction form.

Thank you.

UPDATE: Lets close this. Found the way.

Was not as hard as expected, we did simply by writing the right code in the form file.