Hy all can we customize or add a field in the related item sublist on item -> Webstore subtab -> related item ?

By that you mean Related Item Carousel?

Yes @Mohammad Sharaf Ali

Just add the field in fieldset and use it in template

Actually, I want to change online price in related item carousel

Simply add the specific price level you want to show instead of online one.

Or if I want to add some extra information just in the related item carousel i.e you will get 50% on this related product if you buy this(x) product

Check the SuiteAnswer on Strike-Through Pricing if it can be used there