Everything is marked as unread.

[roosendaaldegekste.nl]

After I coverted a Vbulletin 4 board to SMF 2RC4, every topic is marked as unread for any imported user.

Can this be undone?

[Norv]

The information whether a user read a topic or didn't, is not converted. So, I'm afraid the answer is no.
One could mark as read all topics, and start from there, if that is really wanted.