You need to add a context for that directory, and set "php_admin_value" there.
For example: if directory /webroot/squirrelmail/plugins/squirrelspell/ was accessed with url "/squirrelmail/plugins/squirrelspell/" then define a context with URI: squirrelmail/plugins/squirrelspell/, location: /webroot/squirrelmail/plugins/squirrelspell/, and set "php_addmin_value safe_mode 0" in "Apache style configuration"