How can I hook into an entityform? So that i could create a custom dropdown which is populated from a database
You can write a hook_form_alter()

function hook_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    dpm($form);
} 


After you cleared the cache you can print the form ID or just alter the form right away.

I suggest to check what the form ID is and then add:

function hook_form_FORM_ID_alter(&amp;$form, &amp;$form_state, $form_id) {
    dpm($form);
} 


To alter it. (Make sure to clear cache again)

in both instances hook is to be replaced by your module name. When you want to use the dpm() function you'll have to install the Devel module.

I hope this helps!