Welcome to the Board!
Custom number formats usually break up the numbers into three categories, negative, positive, and zeroes. I do not know of a way you can do what you want for specific numbers using a Custom Number format.
I would probably opt for a lookup table, or VBA code do what you want (the VBA code can be created to run automatically upon manual cell updating).
If you are agreeable to a VBA solution, what range of cells would you want to apply this to?