You can write a procedure or add a step into IKM, but it can be done exactly as you described this, too. I have just tried and had fun:
1. created a synonym in my source schema for sys.dual and called it MYDUAL.
2. created a corresponding datastore in my model.
3. Created an interface with a meaningful table as a source and UNION ALL with MYDUAL as a source.
4. Set implementation to desired constant without connecting any source columns to the target.
Duplicate the LKM and IKM you are using, create copies , something like LKM xyz (No Source) , IKM xyz (No Source).
Then adust the KM steps accordingly, remove the 'FROM' part and replace with VALUES ( ) , keeping the odiref substitution call in the parenthesis.
So you basically have a knowlede module that accepts no source data, generating :