I turned on Station Options>>Preferences>>Show Hidden Properties and then exported the Step.TS.Mode property to a limits file. I then added a PropertyLoader step and it functioned correctly.
Note that importing a hidden TS subproperty makes an edit to the sequence file, even if done at runtime. Thus if you later save the file, you might end up inadvertently saving the last imported values. This isn't necessarily a problem if the values in the file are always overwritten by a property loader step.
(btw, I tested this in 4.0, but I'm hopeful it would also work in 3.0)