If there is someone who did one of the three required runs for a category, but for whatever reason they are not a registered user of speedrun.com, can I, or someone else, submit their run, while on my page it will say I'm a third party? EDIT: Or can a mod do it maybe?

A leaderboard mod can do it. Personally I'd like to make sure the individual in question was okay with it (as I did the one time I did this myself, https://www.speedrun.com/flashback )

A moderator can do it, but only if the player agrees to it.

But the player in question (Gen8603) is not a user of speedrun.com, so how do we ask them?

On YT or Twitch or any other social media they have

Thanks.