@Ukbaz,
for 1.7 just add the following code in the template themes/yourtheme/templates/catalog/product.tpl where you want the button to appear.
<div class="contact"> <a class="btn btn-primary add-to-cart" href="{$urls.pages.contact}">{l s='Contact us' d='Shop.Theme.Catalog'}</a> </div>
And then add css style for the div .contact if needed or change the add-to-cart style if you want a different style rendering for the button.
ie:
#product .contact { padding-top: 25px; }
Don't forget also to translate the sentence "Contact us" if you use a different language than english.