So, just prefix the nick with ! and it should work like a charm. If this doesn't produce the desired result, there's always the +host and -host commands

### adduser [!]<nickname> [handle]
Creates a new user record for a user on the channel, using their current
hostname. It's similar to a user /msg'ing the bot 'hello', except that no
information is sent to the user. If the bot already knows someone by that
nickname, and the user on the channel doesn't have a bot record, then it
does the equivalent of an 'ident' for that user--except that, again, no
information is sent to the user.

If the user is using a different nickname than the bot normally knows them
by, you can specify their "handle" (the nickname that the bot remembers
them by).

If you want to add a user using a static hostmask, prefix their nick
with a '!'. For example:

.adduser !Lamer

See also: deluser, +host, -host, +user, -user