Yes! The enhanced options like the video lectures and Smart Online Flashcards can be added to your account at any point during your enrollment.
To purchase the Video Lectures after enrollment, visit Practice-->Video Lectures and click "Add Videos". There you can choose to purchase the video bundle or individual videos by subject or subtopic. The price of each will be denoted to the right of the video title and length.
To purchase the Smart Online Flashcards after enrollment, select Practice-->Flashcards. At the top of the screen, next to "Flashcards" you should see a banner that says "Trial Version / Upgrade". Click the banner, then select "Upgrade Now" and proceed to checkout!