No need for this. I have found the solution for you. Double click between two markers to create a time selection at the right or left to start with and then just shift click at the marker or ruler to finish. It will extend the selection very fast the way you wanted.
Don't forget to use snap enabled! Hope that helped.
Greetings
Eli
BUT in that context it would also make sense to allow double clicking between markers to extend a time selection. Example would be: Double click between marker 1 and 2, then shift double click between 2 and 3 to extend the selection from 1 till 3. This would be a logical workflow. At least for me.