I know [name]Willa[/name] is the girl equivalent of [name]William[/name], but can it also be for [name]Willard[/name]? [name]Just[/name] making sure…
Why not? I think [name]Willa[/name] could honor any [name]Will[/name]- name. 
I agree with ashthedreamer. [name]Willa[/name], will go with any [name]Will[/name]-name.
[name]Willa[/name], appears to be closer to [name]Willard[/name], just by first glance.
People use it as “the female form of [name]William[/name]”, probably because there are so many more [name]Williams[/name] in the world than any other male “[name]Will[/name]” name. I just read that “[name]Willa[/name] originated as a short form of [name]Wilhelmina[/name].” I think that means that it can be the female form of Any male “[name]Will[/name]” name. And the pp is right, by the way it looks, it seems to actually be closer linked to [name]Willard[/name] than [name]William[/name].