Displaying the same product for several categories

Log in your administrator panel and click on the Manage products link in the CATALOG side box on the left.
Click on the link of the desired product.
Open the Secondary categories tab.
Select the desired categories in the Secondary categories select box.
Click on the Save changes button.

NOTE: You are able to add Secondary categories for a product using the import option. The description of the correct format can be found in the Imported fields format article.