if you just want to associate them directly, and not make them default, you can add them to
/usr/share/applications/mimeinfo.cache
(system-wide), or
~/.local/share/applications/mimeinfo.cache
(per-user).
edit: using xdg-mime, as geekosaur suggests, might perhaps be more robust. In this case, you would want
xdg-mime install [--mode mode]
[--novendor] mimetypes-file
Adds the file type descriptions
provided in mimetypes-file to the
desktop environment. mimetypes-file
must be a XML file that follows the
freedesktop.org Shared MIME-info
Database specification and that has a
mime-info element as its document
root. For each new file type one or
more icons with name major-minor must
be installed with the
xdg-icon-resource command in the
mimetypes context. For example the
application/vnd.oasis.opendocument.text
filetype requires an icon by the name
of application-vnd.oasis.opendocument.text
to be installed.