The conversion from Use Case Points in Function Points is a very delicate process.
It depends how the use cases have been written. There can be a use case that contains more elementary processes and another one that constituites a single elementary process.
To find a rate of conversion we have to establish some restricted rules to build the Use Cases.
In any case the question is : if we want the function point why don't we count them?
A good counter can have a productivity of 100 FP an hour if the use case are written well and there is someone that can clear some doubts.