Adding it to phrases is done in the templates like this (example from the navbar):
HTML Code:
<phrase 1="$bbuserinfo[username]" 2="$vboptions[bburl]/member.php?$session[sessionurl]u=$bbuserinfo[userid]">$vbphrase[welcome_x_link_y]</phrase>
To be able to use them in notices, you'd need to write a plugin, probably using the nocies_noticebit hook location, to make these variables available for use.