When creating a custom step type, you can check "Designate an adapter", and use the Sequence Call adapter. If you would like to make the user unable to change this property, you can disable the "Can Specify Module" property on the "Disable Properties" page.
Warning: If you ever want to change the parameters or the sequence call you are making for all steps, you must load ALL of the sequences that use this step, check the "apply changes in this dialog to all loaded steps of this type" box when you make the changes. This means that you should be very careful to make sure you have your step type designed properly before you begin using instances of it. The reason this is the case, is that the module is a property for each instance of the step, rather than the step type. We are simply giving a default value. By disabling the specify module dialog, we are preventing the user from being able to change from the default.
Allen P
NI