The thing about FormMaker is that it doesn't actually add capabilities that you can't do in Acrobat. It just brings Acrobat into InDesign, so to speak, so that pretty much everything you can do to an interactive form in Acrobat you can now do in InDesign. The advantage, of course, is that you can create the entire form in InDesign and not have to keep going back to Acrobat to make changes and add interactivity to your form, it can all be done in InDesign.
Now, to answer you actual question: Essentially, creating form fields on the fly would be something that can be done in Acrobat, and so it can be done with FormMaker in InDesign as well. But, it does take a good knowledge of Acrobat form scripting!
So if you are able to write such a script in Acrobat – i.e., something that responds to a mouse click on a button and adds extra form fields in predetermined places – then that script could be inserted in InDesign.
I offer some basic Acroform scripting help for anyone who purchases an annual license for FormMaker, so if you were to do that I could point you in the right direction to put together such a script.
Otherwise, there are some excellent people on the Adobe Acrobat scripting forum, and they might be able to help as well.
Hope that answer the question somewhat!