I like the concept of the idea but I don't think it would work in practice. There are too many variables to consider here. There are certain donor perks that escape this problem, such as being able to skip tasks for free, extended block lists, being able to choose your task. The ability to extend the tasks, at least for most players, is a one time investment. I think it's too niche of a need for players who only sometimes want to extend their task.
These donator perks would still be there with their purpose and this suggestion too, since it's in a different purpose.
The main purpose of this suggestion is for slayer streak hunters keep their streak without having to pay 10-30 slayer points every time to skip task, and when they get out of points, having to do 100% the kill amount they get from slayer master, also avoiding skipping tasks for free but resetting current slayer streak.
It may have almost the same purpose as for paying 10-30 points to skip current slayer task (depending on donator status) but, my suggestion is to bring both options to Increasing/Decreasing current amount of slayer task by 25% paying 5 points per time. (example) (points cost also maybe could be based on donator status).
- An example can go for when i'm looking for Kalphites tasks to hunt in for weekend pvm hunt event and I want to get 25% more kills than I already get in the start. (Resetting streak)
- Other example can go for when I get 70 Black Demons to kill but I want to decrease the amount of kills by 25% since it takes long time. (Do not wanting to reset streak).
Instead of paying 10-30 slayer points to skip the task and try another one, I would do the task but I would have an option to increase and decrease the kill amount, but paying less than for skipping it since I would eventually complete the task.
I think something that could be added that works somewhat in the middle, is that for tasks you have extended, the slayer master asks if you would like an extended version of the task or a normal amount, kind of like when they ask what kind of tzhaar task you want or if you want to repeat your same task from the slayer cape perk. (Or for the perk to just be toggleable) In regards to lowering the amount of kills in a task, I think the donor perks of having an extended block list / skipping tasks are more than suitable for not doing tasks that you don't want to do. Something that I do think would be cool would be to have the ability to extend more tasks than just those available
The idea you said also sounds good, but would be better if also bringing option to decrease current amount of kills you want in.
at all, my idea of the re-work in there would make it more professional and in a different tab.
The question here is not about skipping/blocking tasks 'you don't like to do', but, keeping the tasks you do not like to do but you need to do because you are on streak and do not want to spend too many time and points only to skip it and get another one.
Hope my explanations and the main purpose of the idea are making sense. 
