It depends what you mean by expanding the number of boots...
When you start the game for the first time (I assume you are talking about Pro Evo 5?) you will have several styles of generic boot, which you can edit the colours and design for, as well as several pre-selected Adidas branded boots, the old style Predator Pulse and F50 and an Asian market only boot called the Patique. You can change the Adidas boots for another Adidas style but you can only swap one directly for another.
With the editable boots, you can have four different colour variations of whatever you choose to design.
It is also possible to replace the slots occupied by 'official' Adidas boots, with an editable type boot, which would essentially give you the same boot in four different colour choices, and subsequently a larger choice of boots.