I guess you did not get my point. On the desktop clients I did enable subscriptions, but for a very large list of folders.
I do not want to see all of those folders on my N900, so what I am trying to achieve is a local subscription in the Maemo E-Mail client.
On my Nokia 5800XM this was an easy task - my guess is that I'll need to edit some config file on the N900 and hard-code the local subscriptions there (which is not a big deal for me). Just point me into the right direction and I can figure out myself.

Thanks!