there is a My Oracle Support article somewhere that describes how to add a custom formula function in Fusion.
This however is not supported in a cloud environment and in most cases not necessary.
The functionality has been improved a lot since eBS and most things that you'd normally use a custom function for can now be done directly in FastFormula.
New features include: changes to contexts, arrays, loops, nested formula calls, working storage, ...
Check out the Fusion Fast Formula guide that is included in the latest documentation set.
Hope this helps
You need to have database access and then only you can create formula functions. Since this is not possible in cloud, you will need to use existing database items. There was a similar requirement to create formula function in this forum thread and later it could be achieved using DBIs.