Disable but view an account in Mail

I recently imported all my mails from my PC to my new iBook, finaly. I do have all my e-mails since 2001 and I would like to keep them.

Over the last years I've had a few mail accounts, some of them are still in use, some of them are closed, don't exsist anymore,... I added all these accounts in the Apple Mail application. When I added the old accounts I had to use a dummy POP server and password because the POP field isn't optional (why not?).

Adding the imported messages to the accounts was no problem, drag & drop and all the messages are in the right mail account.

Now, every time when I check my mail manualy (the "include this account when automatically checking" is turned off) I get an error because the server / password combination is wrong (obviously).

The only way to bypass this is by disabling the old accounts but when I disable these accounts I can't see these messages anymore and the search function doesn't find these mails.

I could put the messages in my active account but then I loose the original account and I would like to keep this meta data (so I can see where the message comes from).

Anyone has some idea to disable an account and still view the contents?