Hi Max,
Yes, Path and ST-Sim both allow for loading initial conditions from excel. The limit would be the number of records possilbe on the version of excel you are working with so if you have excel 2007 or later that would be 1 million records.
Note that at the moment. TSD is not set for individual records in the initial conditions property for Path or ST-Sim. You can randomize TSD by stratum. In the future it is conceivable that your suggestion of randomizing by state class could be implemented or also the ability to specify TSD values for individual records in the initial conditions property. Note that in Path and ST-Sim TSD is referred to as Time Since Transition (TST).
Regards,
Leonardo