you simply have to attach the weapon by a tgirrer into the hand you wish.
just import the orc and the weapon in your map. than make a trigger with the action: create special effect on right\left (cou can chose) hand of a unit.
the specialeffect path yould be the path of the spear.
Here is the trigger: Event: Map Initiation Action: Special Effect - Create a special effect attached to the hand,right(or left) of (whatever unit u choose could even be a casting unit or triggering unit etc.) using (model path)
But you may want it for all, so base a ability off sphere and change the attachments to 1 and delete all sphere first second and third and add one which is hand,right or hand, left then give it to the unit type. You will have to set the projectile art to NONE.