linking custom transformations to a page type

I just created a datasource (ascx) and want to use a list web part, for instance basic repeater, to display the data. To do that I created a transform (ascx) and want to use it in the repeater, but I cannot select it.After searching the documentation I found that I need to add a transform to the page type. I can add a new one, but it expects the code body to be in Kentico. So my question is how can I add a transform that is located in a file not in the database to a page type?