A button which runs Copy INCLUDEINLIBRARY="My Library" (with the name of your library) might work, if you select what you want to add and click the button. (For most folders, if you right-click or shift-right-click them, there may already be a menu for adding them to a library. That won't be the case for drive letters, where you'll need to make a button specially.)
That will sometimes let you bypass Windows restrictions on what it lets you add to libraries using the normal UI. It's possible it will still detect it's a removable drive and block it from being added, and it's also possible it will add the folder to the library but with unexpected results or unwanted behavior later on. (But I have forced network shares into libraries using this method, and everything I used seemed to work OK.)