To create a new test step, you would also use the "im createissue" command. There is a type for "Test Step". To add a new test step to a test case, you would first need to create it using "im createissue", then add it to the "Test Steps" relationship for your Test Case. For example, if I wanted to add the Test Step with ID 573 to the Test Case with ID 580, the command would be im editissue --addFieldValues="Test Steps"=573 580.
Note that I am using Integrity 10.7. For versions prior to 10.6, the command would be im editissue --addRelationships="Test Steps"=573 580