Is it possible to add new tasks to a sprint in the current sprint?

Question

Agile

We are just trying to implement Scrum in our company (not IT), sometimes there are problems and disputes. For example, is it possible to add new tasks to a sprint in the current sprint? Or is it possible to make a certain "reserve" of tasks in case one of the employees has a blocker in his work or all his tasks in this sprint are completed (and colleagues do their tasks perfectly without his help)? Can sprints within the same project vary in time?
Maybe someone has been using Scrum for a long time and will be able to tell from personal experience.
Thank you!

Answer 1

The direction (tasks) of development change based on the results of the sprint, so until the end of the sprint, the sprint itself does not change. This is a time-fixed stage, roughly speaking, we work for a week, at the end of the week we evaluate our work (no matter how much we did or did not have time) and try, based on what we did, to adjust the direction of development.
Sprints are also fixed in duration so that you can evaluate the performance of sprints with each other.
Of course, you can of course do as you like.
PS
In the vast majority of cases, under the guise of an agile methodology, they use the usual waterfall, with the only difference being that the development stages are called sprints for some reason, with a detailed task plan for a dozen sprints ahead.

Answer 2

In fact, in a sprint, only the goal cannot change, and not the set of tasks for its best achievement. So there is flexibility with the set of tasks performed.