Hi I was thinking to use a knife with the animation of attack and all but use a back shealting for the scabbard and knife position so would be on the back ... Is that possible?

That way I could replace a knife with a spear...

up.

It is Possible but it would require a HUGE bunch of Edits/Additions to the Behavior Files and you would have to give all of the Weapons you want to use this setup a New Keyword so you can differentiate them from the default weapons in the Animation Manager.

Doing this I have animated and added to the game 1H Spears, 2H Spears, Throwing Potions among a few other things.

Editing the Behavior Files is a bit tedious with the tool available to you guys but it is possible to get functioning Custom Combat Sets made up of the Duplicated Default Animation Events with the Mix and Match setup -> what I am not so sure of is if its possible to get the game engine to treat a Dagger during the Equip/Unequip Events to actually sheath on the back my guess is it would revert to its hip position no matter what animation you played for the Equip/Unequip as those are the Hardcoded postions for 1H weapons, What I do is I use the Float Tracks on the Weapon and Shield Nodes to make my 1H Spears invisible when Sheathed otherwise it looks a little funny.

Ok I hope then that they will add spears in next dlc so we will have possibility to have finally them without any work around ....

There's this mod:
http://skyrim.nexusmods.com/mods/15658

I have no idea how it works, but apparently it's possible. Maybe there's a node/bone specifically for each weapon type to attach to and the mod moves them? (Sorry if that's completely wrong, I haven't done much with character skeletons or animations yet...)