Is it possible to add personal folders not located in the Zbrush directory into Lightbox, or have the ‘Load’ button in Tools/Subtools point to personal folders?
If you are on Windows then you can create a shortcut to the folder and place the shortcut in one of the ZBrush folders. Lightbox will show the shortcut and you can use it to open the folder you want.
You can create a shortcut by right-clicking the folder in Windows Explorer and choosing Send To > Desktop (Create Shortcut).