You cannot configure a infopath client event without a task action. For your scenario, you could just configure a single task action(ex: submit) and not bind it dropdown to make it look like a formal submission.

Good news, it can be done!  I am assuming that the step in question is in a InfoPath client event.  Here are the steps:

• 
  
• Open InfoPath in design mode from the K2 InfoPath Integration wizard
  
• In the Data Source task pane, add a new string element to contain the action for this step 
  
• Give the element a default value of the action you want to take when the user submits (this must exactly match one of the actions in your client event)
  
• Close your form and save changes
  
• When you're going through the InfoPath client event wizard, specify this newly created field as the task action field
  
• Answer no when it asks if you want it to create a control for the field.

When the user submits the form the action you used for the default value of this field will be used to action the task.

David