You could do that, in fact it would make the spell more powerful since you are now infusing it with your own energy. You can customize a spell anyway you want.
NO! Satanists cannot use Wiccan spells ... Create your own spells first and second learn that you have to choose left or right handed path... There is a coven that is left handed path... And btw you say you are a mix of everything so ... -.-
A spell is simply a way of directing energy and will to accomplish a particular purpose. It is a magical practice, not necessarily a religious one. So there is no reason that someone could not use a spell crafted by a practitioner of another path. You might need to modify it a bit to make it more personal and appropriate for your purpose, but other than that there are no limits.
