Use a continuous transfer fcn in a subsystem call from Stateflow

I want to call a continuous transfer function from Stateflow subsystem function call but MATLAB restrict due to:

Only constant (inf) or inherited (-1) sample times are allowed in function call subsystem

I appreciate if someone could provide a simple solution other than discretizing transfer fcn.

[Doug Eastman]

How would you expect this to behave? Do you want it to run for a certain period of time and return the output? If you could provide more information about the application that would be helpful.