Hi, am assuming that you don't want this extra lesson to appear on student and teacher timetables but need it for registering the students who turn up to this extra lesson? If so, you could add a new subject and create a block in the model screen. Add the classes to the block and assign 0 lessons. You can create a derived block if you want to mix the classes up or set up blocks in each year group. Then assign staff in the Classes screen and send across to SIMS via SQL. The classes will then be available in the Curriculum Assignment by Scheme screen and the students can be assigned. These classes will not appear on student or staff timetables but can be found in Take Register if this is what you want to do.
I hope this helps anyway.