Custom field on Product or any other objects

[hydromel2008]

Hello, 

I developped a while ago a shop using prestashop 1.6.1.x, it's using overrides and custom fields on many entities because I needed to write connectors that would translate our old, but trusty, custom AS400 erp to prestashop. A lot of work have been put into this to be able to seamlessly integrate prestashop to our current workflow. The core file were untouched in the process, I only extended using override system. 

NOW where this is getting interesting is that I need to somehow, do the same thing using prestashop 1.7.x, because I don't want this webshop to be outdated. From what I read, overrides are evil and everything should be contained in a plugin. 

But, how the hell am I will I be able to add fields to products, categories, vendors, suppliers.

Is there a kind soul that could help me or give little insights on how to accomplish,properly, custom fields ?

[hydromel2008]

I know prestashop 1.6 inside out, but 1.7 seems like a totally different beast because of the underlying symphony framework and all the breaking feature things.