Q:

Sitecore: How to add a set of "fields" to a rendering

In Sitecore, how would I go about adding a couple of fields to a rendering (.xslt)?

Changing the "template" of the render, makes it so it is no longer a proper rendering.

The goal is to create a rendering, the user can add to a placeholder in the page editor.
Edit:
Currently im trying the same using sublayouts (.ascx)
- The sublayout is of a template of (..sublayout), with a "custom" template added to the "Parameters Template" field.

A:

Take a look at the Presentation Component XSL Reference on SDN.
If you are changing the template, you also need to change the rendering control (XSLT or ASCX depending on whether it is a Rendering or a Sublayout) to display the new field you have added to the template.
For example, if you add a field called "summary" of type "Single Line Text" to the template, then you need to add following to the XSLT
<sc:text field="summary"/>

Section 4.1 has a list of the different field types.
If you are using an ascx control then
<sc:FieldRenderer ID="scSummary" FieldName="Summary" runat="server" />

Just adding the field to the template will not cause it to appear on the site (unless, of course, you coded your rendering/sublayout to do exactly that).

