If the alias email does not contain a dot (e.g. foo@xxyyzz.com)
nano .qmail-foo
./real.name/Maildir/
If the alias email does contain a dot (e.g. foo.bar@xxyyzz.com)
nano .qmail-foo:bar
./real.name/Maildir/
Just replace the dot with a colon in the alias filename. This was added as a security feature to prevent people using the dot for directory traversal.