New Animations With Mechanim

Is it possible to create new additional animations to an object that has animations designed through a 3rd party program through Mechanim?

Yes. Assuming the model is bipedal, make sure it’s set to Humanoid (under Rig in the Inspector). Drag it into your scene and click on the model in the Hierarchy. It should have an Animator component attached, with an Animator Controller. If you double-click on the Animator Controller, it will open the Animator window and you’ll be able to add new animations to your model by creating new states and dropping animations into them.

I recommend you watch this video, it’s very good and it’ll make everything clear: Unity 4.0 - Mecanim Animation Tutorial - YouTube