Official comment
Thanks for your feature request! It has been added to our list of projects, but I don't have an ETA at this time.
Here's is one method of doing this now:
This process will enable you to have a pop up form that collects the data, then returns to the same page the button is on. Change step 3 if you want it to redirect to another Thank You page.
1. Copy the URL of your published Page.
2. Create the Ontraform that you want to pop up.
3. In the Settings of the Ontraform, paste the URL of the published Page in the Thank You page section. This will return the contact to the same page the button is on. You can also use any of the other options if you don't care if the contact is returned to the same page.
3. Edit the published page and add the button from the top section (not the Submit button that is lower down).
4. Edit the button. Click on the Link icon and choose Pop up Ontraform at the bottom of the link dialog. Select your form.
5. Publish the page again, and test.
Here's a 1:30 video that shows this process:
http://screenshot.ontrapedia.com/public/fhagan/b24f0f77-8026-48d0-a4c5-9bdf63a5e3881629932498.mp4