How do I add additional animations to a 3D model I purchased that already comes with animations?

Unity Engine
1

Title

I purchased a 3D model from the asset store that comes with 2-3 dozen animations but not all of the animations I want. The asset comes with .anim and a few .FBX files. I am on a Mac and I already tried importing the FBX file into Blender but it came out looking wrong. Perhaps I just don’t know how to use Blender, I’m a programmer, but what is the best way to add animations to this model?

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The link provided takes me straight to the main page of the asset store not the direct link to the model ypu are using.

Is the model rig type set to humanoid? It needs to be.
Also all the animations that come with it will need to be set to humanoid and assume the avatar definition from the character rig.
Once the model is humanoid, you can set up a compatible humanoid rig in a 3D animation package and create the animation you want.
Export as fbx, import as humanoid rig and assume the avatar definition from the character.