Question about modifying files displayed the solution explorer

Question

This may sound like a ridiculous question to be asking since it isn't one of necessity, but I like to keep my dev enviornment as neat as possible so I'm going to ask.

I have a form that consists of a lot code.  For better organization I have created another document and using the partial keyword to declare the class I have split my code for the form into two logical sections.  Technically, however, it could be said that it's divided into three.  Upon initial creation of the form, VS makes a form.cs file as well as a form.Designer.cs file.  To follow this convention, I named my new code file form.InsertItems.cs.  I want my new code document, which is part of the form class, to appear with the expandable group underneath the form.cs file in the Solution Explorer (just like the form.Designer.cs and form.resx files).

Can I do this? If yes, how?  Thanks for any response.  It will be appreciated.