I don't know if I can answer that one very well for you. I am not aware of any documentation of X-CART architecture and data structure - so I just look through the code and database to figure out how it is built - just doing it in a piecemeal way to figure out whatever it is I am trying to accomplish. I suspect every one who has any success making changes does it similarly, or the x-cart companies may have internal documentation that they share to teach one another.
I normally look at the names of the php files and look around for the appropriate queries or variables and go from there. For example for products, I start at products.php. For an individual product, I start at product.php. When I am looking for something in particular - I grep for the variables names in question, and just look at the code to find the queries that are pulling the data from the database, and the php that is organizing the data and arrays to pass through to the templates with smarty.
I think if you look at
this thread, and are able to understand the code changes made, you should be able to figure out how to modify queries to get extra data, and how to add that data to the products array. Unfortunately - it is a little hard to follow because of all the different inputs, and it is not exactly what you are trying to do - but if you can grasp the principles of the query (mysql), php and passing the data through smarty to the template - then you will be able to do a lot of things with it. If you are pulling in new data - you also have to know where it is in the database, and how the tables in the database relate to one another - so you can write a query to get it.
Also - if you have the persistence - here are a couple more threads with examples of msql->php->smarty->template. They have to do with extra fields - but the methods are the same - getting the data in php, storing it in arrays, and passing it to the templates.
http://forum.x-cart.com/showthread.php?t=46059
http://forum.x-cart.com/showthread.php?t=48320
If you can get a start and ask questions here - hopefully I and others can help you. Or - a quicker way might be to post in the Professional Services forum, and just ask someone to do the php for you, and provide you with the smarty arrays, then you can get your site styled and released, and come back and look at the php later.
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