How To Pass A List Of Values To Compare In A SQL Function In SQL Server 2008?

I have an SQL Function with the following SQL within:

SELECT StockID FROM (SELECT DISTINCT StockID,
ROW_NUMBER() OVER(ORDER BY DateAdded DESC) AS RowNum
FROM Stock
WHERE CategoryCode LIKE @CategoryID) AS Info
WHERE RowNum BETWEEN @startRowIndex AND (@startRowIndex + @maximumRows) - 1

I have a Parameter @CategoryID - however I need to take in a category ID such as "BA" and translated this to a list of Category IDs such as "IE","EG" etc so my WHERE clause looks like:

WHERE (CategoryCode LIKE 'IE' OR CategoryCode LIKE 'EG') AS Info

I have a Lookup Table which contains the "BA" code and then all the real category codes this means such as "IE" and "EG".How do I have the CategoryID expand to multiple "OR" statements in my SQL Function?

At the moment the query as shown can cope with one CategoryID such as "IE", this is done as I want a category page such as category.aspx where a parameter "BA" is passed such as category.aspx?category=BA and this page will list all items with the category codes "EG" and "IE".

The reason I need this is there is a "parent" category code which has multiple "children" category codes which are different to the parent code. I am using ASP.NET and .NET 3.5 on the front-end if this helps.