Yes, if I have a bow with 25 SSI and I enhance it with Ash then one of three things will happen.
A. I successfully enhance the item and add 10 SSI
B. I fail but the item isn't destroyed
C. I fail catastrophically and the item is destroyed
If I craft a bow with Ash, then it will always have at least 10 SSI. Not sure if anyone has a chart out with the odds of successfully enhancing an item. It seems like the more mods a weapon has already the higher the odds of breaking it.
When SA comes out with imbuing, then you will be able to add mods to any weapon that hasn't been enhanced or made with special materials with out risking destruction of the weapon (if you fail, you loose materials). There are some other restrictions, but until SA gets released all things are subject to change. After SA is released then it will be subject to being nerfed.