You can do this, and it's generally my suggestion for situations where a person was particularly bad, especially in post-game chat. Or if somebody adds you in game and starts harassing you. Situations like that where there's no other means of reporting them.
I don't really recommend submitting extra tickets to support to report players in situations where you could have reported them after the game. For one thing, there's a decent chance somebody else on the team reported them, so the ticket would just be redundant. For another thing, if the player is doing that sort of thing regularly, then they're going to be caught up by the system eventually. Missing a report in a single game isn't going to suddenly let the person run around with a clean slate.