I'm guessing you know about ctrl+d/cmd+d and are wanting to make arbitrary selections via the keyboard?
I wrote a plugin back in the days called AutoSelect, which adds a sticky selection mode.
It's an EventListener which implements the on_selection_modified hook.
When modal, any non caret selection, ie one that contains text, will
stick as you create selections.
At the end you hit a command to merge all the selections.
What if you want to store empty selections? There's facility for that too but it's not automatic, you have to manually mark the selections.
There's no bindings set up there, so you'll have to set em up yourself. There's a template in the source code.