How to add a checkbox to a Content Type and act if box is set to true or yes
You can add a field in a number of ways:


Edit the content type and add a Boolean field through the UI
In your .install file as you already mentioned
Using hook_form_alter() to add a #checkbox.


If you used option 1 or 2 you can pick up whether the field is checked or not using hook_node_presave() or hook_node_update().

If you used option 3 you'll probably need to add a #submit value to the form which calls a custom function in your module.