It is not possible for them to be "created" dynamically, but it is to show existing form elements dynamically.
If you want up to 10 "Foo"s to be added in an order, you would need to add multiple "Foo" dictionaries (in the current release... this is being addressed in a future release).
This means you would have "Foo_1", "Foo_2", ..., "Foo_10" dictionaries. Define that these fields are hidden, either in a rule or their display properties. It is common practice to have a check box "Add another" in these dictionary, and a Conditional Rule to show the next dictionary. So there would be a Conditional Rule "Foo_1 checkbox show Foo_2", it's condition would be the "Foo_1.showNext == true", and it's action type would be "Show Fields", with dictionary "Foo_2", and "All Fields".
Or you use JavaScript as you suggested with a button. You were close to finding what you were looking for! "serviceForm.dictionaryName.setVisible" is the call, so "serviceForm.Foo_3.setVisible(true)" would work.
As you can see it's complex, and assumed a relatively small maximum number of entries. If you can wait, do so, for new functionality coming in a later release.
(There is another alternative that I have not seen implemented, but I have considered. Write pure JavaScript to add additional form elements to the page DOM. This potentially breaks during a product update, so do at your own risk! Then have JS that reads these values and writes them as a concatenated/formatted string in an existing dictionary - maybe as XML. Then in your Adapter, assuming it is CPO, you can process this formatted string, and do what you like with it.)
