Can a Custom Field be added to a Contribution Page or Financial Type?
Under Administer > Custom Data and Screens, Create your Custom Fields 
Under Administer > Profiles, Create a Profile that has your Custom Fields in it.
Under Conrtibutions > Manage Contribution Pages, select the Page you want it to appear on. Under the Include Profiles tab, add that Profile. You can add it at the top or bottom of the Contribution Page.