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You can use grip editing to stretch the boundary and the hatch, and it should be associative, will adjust to fill the new area of the boundary. Please note that, you have to move at least two connected lines from a common endpoint if your boundary is LINE objects.
Or from the point you want to extend select the option for "point" or "object" selection and the hatch will move to fill the new boundary object. It will use the currently selected "island detection" mode so it may be removed from the original boundary if the new one is completely surrounding it. If this happens just delete the original boundary or change the island detection mode.