Add a local dir to icon theme directory list?

[leon]

Hi all,

In the Icon Theme Specification, it states:
...
By default, apps should look in $HOME/.icons (for backwards
compatibility), in $XDG_DATA_DIRS/icons and in /usr/share/pixmaps (in
that order). Applications may further add their own icon directories
to this list, and users may extend or change the list (in
application/desktop specific ways).
...

>From the above infomation, I still cannot figure out how to add
directories to the list. Any suggestion? 

Thanks.
-- 
Leon