I see have asked this question a while ago, hope it is still relevant to you. Could you elaborate your use case a bit more?
How would you like to set up the mode? Do you want to set up the problem using StructuralModel approach for analysis and then instead of calling solve, do you want to call some other function that would reduce the model and return an output that is not the solution for a specific loading?
What all do you expect as a part of reduced model, like Component modes, Fixed-Interface modes, degree-of-freedom mapping to boundary etc.?