Can I just upload an existing fillable pdf form that id like to use?

Hi,

Yes that is possible, you can upload your existing fillable PDF form to us and you can auto collect data from it. 

Here's the Import Tool that you can use: http://www.jotform.com/adobe-formscentral-alternative/ 

Scroll down and see the Import button at the top right page.

Please do take note that the fillable PDF form won't be editable in our form builder. When the fillable PDF form shows up in your account, you can download it again.

You'll then have an option to auto collect data from it.

I hope that helps. Do let us know if you need more information on this.