So, the way I've always played you can't add a draw 2 on a draw 4 but I've seen plenty of other people do it. So I'm wondering if you can or can't?
Be extremely cautious about what you mean here.
If you're referring to the "Progressive UNO" House Rules variant, then it depends on exactly how you've defined the house rules.
If you refer to standard rules, nothing prevents a +2 from being played on a +4, provided that it is your turn and you match the color chosen (matching the number is not possible, since +4 and +2 are not the same number).
You can as long as it's your turn and the color of your Draw 2 matches the color specified by the player who played the Draw 4. In standard Uno rules, the player after the one who played the Draw 4 will draw the 4 cards, and the player after that one gets the next turn. So, you would need to be the player next to the player next to the one who played the Draw 4 to play your Draw 2 (again, assuming it's the correct color). The player after you would draw 2 cards, and it would be the next player's turn.
An example from @JoeW: player 1 plays a Draw 4 and calls red, player 2 draws 4 cards, player 3 plays a red Draw 2, player 4 draws 2 card and play continues as normal.