I create a column "LookUpValues" of type LookUp in
Site A but I'm trying to add from existing site columns in
Site B but it doesn´t show up. Is it possible to do it? Any ideas?
It is possible to use the content type from one site to another site using Content type hub.
But to use the content type from one site to another site, you need to do some configurations. Follow the steps given in below articles to achieve this:
If the Site A and Site B are in the same site collection, you can create a site column with lookup type in the root site of the site collection. The site columns are available to all sites in the site collection.
Learn more about how to create a cross-site lookup site column
If the Site A and Site B are in the different site collection, you need code to achieve that.
For example, you can do a cross site collection lookup and populate it as a dropdown control in your SharePoint application using jQuery.