Has anyone created a 'shadow' form?

Sounds mysterious, right?! Basically what I'm after is something some old co-workers did long ago where we created a form (in Eloqua) and then basically mapped it to a lead capture form on a 3rd party content syndication site. So instead of having to upload lists, anyone who submitted would automatically be submitted into the marketing database.

Has anyone done anything similar in Marketo?

Thanks!