[Dovecot] multiple mailboxes in imap

[einKI.ml]

Hi

I try to setup a mail server which fetches mails from
some pop accounts and presents them through imap (with
dovecot) to my local network.

I get the mails with getmail at this works as it should.
To use dovecot as a pop3 and imap server works too.

Now I am trying to setup multiple mailboxes which then
should be presented to the user as different folders.
I think I have to use namespaces for this (is this correct?)
So I tried to configure two namespaces with the following
statement.

namespace private {
separator = /
inbox = yes
prefix =
}

namespace private {
separator = /
prefix = ns
location = maildir:/data/server/mail/in/%u/ns
}

when i try to access the imap server with thunderbird I can receive
mails for Inbox but when I want to subscripe an additional
folder ns can be seen but is grayed out in thunderbird.

To summarize this can anybody give me a hint or example how to configure
multiple mailboxes which are located in different directories on the
server.

thx