add method from Workspace?

How can I add a method to a class from the workspace?  Adding a class is
easy, just send a message to the superclass.  FileIn does it all the
time, but I can't seem to do it with any variation of the format in a
.st file.

For the curious: the reason I want to do this is to make a classroom
demonstration go more smoothly.

Thanks,
Don McLane