The Contact Form Solution Pack is designed to work in its downloaded state, if you would like to turn an existing for you have into a contact form the best way to do this would be to use Universal Email to add a 'create email message' server behavior to the page to have it email the contents of the form. Manually switching out the form for your own would require some work in the code.
There are some guides on the Contact Form Solution Pack page for getting the security features onto an existing form and also adding the pages to your site. Here is a link to the support page:
contact-form/