Hi Theodore! Yes, this is certainly possible, but the custom fields use a slightly different syntax than regular fields, as outlined in more detail here: http://nationbuilder.com/request_custom_people_fields_on_webpages

So in short, you should be using this instead: 

{% text_field "signup.custom_values.organization", class:"text form-control", placeholder:"Organization" %}

Side note: If anyone else stumbles upon this page and is trying to add custom fields to a ticketed event, it is worth noting that custom fields can only go on event pages that do not sell tickets (and instead collect RSVPs only). If you have a ticketed event, the custom field will need to be placed on the donation page where the ticket buyer completes the ticket purchase.