See that's the main problem with strict scrum. Why can't you add things to the sprint mid sprint? Sometimes things come up that you didn't think of.
There's really no point being that strict. I think two week meetings/sprints are good for keeping focus and as an opportunity to agree short term priorities. But the point is to get work done, not to precisely obey scrum rules.
If the "you" is the team, go ahead. Add anything you want, as long as you deliver what you promised in the beginning of the sprint.
If the "you" is an external person asking for a "quick job", then the default answer is "talk to the scrum master" whose default answer will be "No.". Then they can start discussing if it is so urgent and important that it can't wait 1-10 business days for the next sprint and/or small enough not to disrupt other work.
The rules are there to protect the team. If the team is willing to take ad-hoc work, nothing in the agile rules says they can't do so. The point is that nobody outside of the team can force them to take on extra work mid-sprint and disrupt their planned work.
There's really no point being that strict. I think two week meetings/sprints are good for keeping focus and as an opportunity to agree short term priorities. But the point is to get work done, not to precisely obey scrum rules.