Actually, it seems that OOo looks into the files script.xlc and dialog.xlc 
located in the second directory listed into the Basic path to know what 
StarBasic macros are available. Modifying Basic path does nothing, and can 
broke the usability of installed macros. All macros located into the $(INST) 
directory are listed as OOo macros, and the other ones as MyMacros.

There should be a way to specify macro libraries location shared among users­.

This is what I suggest:
1- For each directory listed into the Basic path
   1.1- If that directory contains script.xlc and dialog.xlc, add those 
libraries into the macro list
   1.2- If that directory doesn't contain script.xlc and dialog.xlc and is 
writable, build those two files by looking for all subfolders containing valid 
script.xlc and dialog.xlc
   1.3- In the macro list, list each location with a different name (OOo 
macros, my macros, shared macros, etc.)

2- When adding a new library to the list
   2.1- If the selected file isn't in a folder located into the Basic path, 
move it the the user basic folder (this is a similar behavior than with 
templates) So all macro libraries need to be in the Basic path.

That way, I could put some macro libraries on a shared directory in my network, 
and with the appropriate script.xlc file, all my computers having that location 
in their Basic path could see those macros. In a similar way, the 
OpenOffice.org macros wouldn't need to be listed in each user profile.