Most units have pre-set attachment points like "head", "chest" etc. If you have a weapon attachment, simply edit the Blood Mage spell "Sphere". Change the model of the spell to your weapon, and then decide where you want it to go in 'Target Attachment Point'. Options include 'weapon', 'hand, right' etc. Don't use the quotation marks though, just type in the word. Then give the new Sphere spell a name (such as Weapon Attachment) and give it to your unit.