I'm working on a package that for some reasons needs to be installed into a "non standard" path, that means neither `$BaseDirectory` or `$UserBaseDirectory`
I added the package's path to the variable  `$Path` so to have the possibility to use Get/Needs to load the package (and it works).
The problem is that, inside that package, I also have a custom palette and stylesheet and they are not visible from the menu (Format->Stylesheet and Palettes, respectively).
On the tutorial/WorkingWithStylesheets there is the following sentence:

    Stylesheet are discovered by the front end in the following locations.
    
    FrontEnd`FileName[{$UserBaseDirectory,"SystemFiles","FrontEnd","StyleSheets"}]
    
    FrontEnd`FileName[{$InstallationDirectory,"SystemFiles", "FrontEnd","StyleSheets"}]
    
    Stylesheets contained in directories are grouped under submenus in the Stylesheet menu. 
    Add-on applications following the same layout also add stylesheets to the menu.

So, I thought it was possible to have stylesheet accessible from my own package, being and "Add-on application" that follows the standard structure, e.g.:

package\FrontEnd\StyleSheets\mystyle.nb
package\FrontEnd\Palettes\mypalette.nb

Of course, if I move the package under `$BaseDirectory` everything works fine.
So, the question: if I install a package in a directory that's not `$UserBaseDirectory` or `$BaseDirectory`, are stylesheets and palettes still visible in the Mathematica menu?
If so, how to? I'm not able to have them.
Thanks and regards,
Roberto