Portal form to Action on Sites

I don't think that Portal Forms are compatible with Sites.  You may have to abandon plans to use Sites, or preferably abandon Portal forms.  Appian will continue supporting Portal, but we don't know for how long.  

You might be pleasantly surprised with how quickly you can rebuild the same interfaces in SAIL, and how much of your process you can throw away as well because you can now use SAIL to do the same thing more efficiently.

I don't think that Portal Forms are compatible with Sites.  You may have to abandon plans to use Sites, or preferably abandon Portal forms.  Appian will continue supporting Portal, but we don't know for how long.  

You might be pleasantly surprised with how quickly you can rebuild the same interfaces in SAIL, and how much of your process you can throw away as well because you can now use SAIL to do the same thing more efficiently.

So is there a way to convert portal form into a SAIL Interface?

There is no automated way, though I believe it is worth the effort to do the conversion manually for anything that you plan to continue using in any capacity in the future.

There is not a direct conversion from a Portal Form to SAIL; you will have to recreate from scratch. That being said, SAIL is way easier to use and provides much more functionality than portal. I'd suggest looking at the documentation on interface design here: https://docs.appian.com/suite/help/latest/working_in_design_mode.html