How to Tweak a Walk (or other library motion)?

I'm learning the animation library, and having success (I'd love a tutorial some time about how the location of tags effect animations), but I'm trying to get them to look better. Specifically, I want to adjust the bend of the toes in the Walk 2 motion library animation. And I'd like to animate the feet to stay flat on the ground. Basically, I want to edit the location or rotation on certain elements at certain times, like one would do in After Effects. I've tried adding transform behaviors to my toes. This works for my arms and legs since they are independent, but when I add a transfor behavior to the toes, they are suddenly pinned and my rigging no longer works as expected. 

Is there a way to animate specific parts of a motion library animation? Perhaps it's obvious and I'm being daft. Note that I have already played with all the settings in the Motion Library, and bending the toes seems like something I'd have to customize.