You cannot add fields to the layout while in browse mode so there is no way to add a button that does this. In Layout mode, you already have a button for adding fields in the status area at the top of your screen. Just drag and drop from the button to your layout and a dialog will open where you can select a field from those defined in your database tables to add to your layout.
It's possible that what you really need to do is add a related record in order to enter a list of data of variable length. With the correct relationships and a portal, this is easy to do. See your other post for details.