open your Resources folder in 3D using the context menu in the class library.
Then right click your worker and select 'Exchange Graphics...'.
Now you can choose Worker_Jack_6.s3d, Worker_Jack_15.s3d, Worker_Jill_6.s3d or Worker_Jill_15.s3d.
The number in the filename is the number of poses. More poses means more memory consumption.
But be aware that the walking worker is only available in V11TR3 and above.
this is possible - but it requires some background:
First of all, the walking workers are animated as sort of a slide show. For each the carrying and the "empty" state of a worker, there is a set of poses that are just switched through. The set corresponds to two steps that the worker makes, so a reasonable sequence should cover these two steps (e.g. moving the left leg -> moving the right leg). These two steps will be animated while the worker walks 1.48176 meters (this is double a common definition of a "standard step length", also findable as a Roman passus).
If you now have such poses, create one graphic group for each and assemble the sequence of graphic groups you want to step through in the 3D properties dialog page "Graphic Sequences" of your worker class.
It might be a good start to analyze the poses of the existing worker and of the four walking exchangable graphics files that come with Plant Simulation.