How to create a shortcut to add/subtract selection in lasso

Although you can't change the shortcut, it is probably best to learn and use the current keys. The reason is important: Shift to add and Alt/Option to subtract are not only used to add/subtract with the Lasso tool, but also for all other selection tools in Photoshop. And not only that, the same keys do the same thing in similar tools in other applications, including many programs that are not by Adobe. In fact Adobe did not invent the Shift/Alt convention — they followed the example set by earlier image editing applications 30 years ago.

So Shift to add and Alt/Option to subtract are actually industry standard shortcuts for adding to and subtracting from selections in general. If you were able to change them in Photoshop, that might slow you down in other applications because you would have created a shortcut inconsistency with all other applications that handle selections.

It would probably be better for you to become familiar with these industry standard shortcuts as they are, so that you can easily work in any application.