Wanting to add function to Common file open dialog

In my app right now I am using the common dialog control for my file open and file save dialogs. I am new to API programming and was wanting to add an extra option at the bottom of the dialog. Not sure the exact format I am going to add, either a check box or some way for the user to select if they wanted to open another file, but for a different purpose, so multi select isn't a good option. I have read some things on bypassing the .ocx and using the comdlg32.dll directly but haven't found anything on adding more buttons, checkboxes, or textboxes. Any pointers would be helpful, Thanks in advance.