We know that it's not necessary, in general, to use a wand for Animagus transformations, as confirmed by the new information written by JK Rowling for Pottermore:
The legend of the Native American ‘skin walker’ – an evil witch or wizard that can transform into an animal at will – has its basis in fact. A legend grew up around the Native American Animagi, that they had sacrificed close family members to gain their powers of transformation. In fact, the majority of Animagi assumed animal forms to escape persecution or to hunt for the tribe. Such derogatory rumours often originated with No-Maj medicine men, who were sometimes faking magical powers themselves, and fearful of exposure.
The Native American wizarding community was particularly gifted in animal and plant magic, its potions in particular being of a sophistication beyond much that was known in Europe. The most glaring difference between magic practised by Native Americans and the wizards of Europe was the absence of a wand.
The Native American wizards had been doing this for centuries before Sirius was even born, and Rowling specifies that the Wizards were aware of the other peoples/countries for that entire time. It can be inferred they were aware of the ability to transform without wands.
Though European explorers called it ‘the New World’ when they first reached the continent, wizards had known about America long before Muggles (Note: while every nationality has its own term for ‘Muggle,’ the American community uses the slang term No-Maj, short for ‘No Magic’). Various modes of magical travel – brooms and Apparition among them – not to mention visions and premonitions, meant that even far-flung wizarding communities were in contact with each other from the Middle Ages onwards.
The Native American magical community and those of Europe and Africa had known about each other long before the immigration of European No-Majs in the seventeenth century. They were already aware of the many similarities between their communities.