how do I fit all the extra material in? This is not a rhetorical question!
Extra lessons for those who want to do it.
This means extra work for you, but you could try some strategy to reduce the work. For example make a list of Khan Academy videos related to the missing materials, put it on your web page, start your first lesson with a (not graded) background knowledge test, and recommend everyone to watch those videos. You could also make some additional exercises for voluntary homework. -- The general idea is to do something once and reuse it every year.
Extra lessons for those who want to do it.
This means extra work for you, but you could try some strategy to reduce the work. For example make a list of Khan Academy videos related to the missing materials, put it on your web page, start your first lesson with a (not graded) background knowledge test, and recommend everyone to watch those videos. You could also make some additional exercises for voluntary homework. -- The general idea is to do something once and reuse it every year.