Can Unity Create Animations On .fbx Humanoid?

unfortunately no, you cannot keyframe a humanoid in Unity right now.

You can use Blender which is free.
http://www.blender.org/

Maya LT is good too but cost 30$/month
http://www.autodesk.com/products/maya-lt/overview

 

in fact yes you can take any free animation from asset store, as long as you have the original file(.fbx, .mb. .3dsmax, .blender, etc..) and convert them to humanoid animation that can be used on all humanoid rig.

Simply set the rig type to humanoid and reimport the file.

 

Just keep in mind that there is no magic with humanoid rig.
Sometime some file may not work because the rig is not fully compatible with Mecanim Humanoid rig, in those case you can see small difference between the original motion and the retargeted motion.

By example if the original motion had more than 2 spine bone, the retargeted motion 1 spine and last spine(waist and chest end) should still be at the same position than the orignal motion but since you have less spine the bending on the character spine may not be exactly the same.