is it possible to use this hack only on some forum id's?

Yes, you can keep certain forums from having their threads split in the acp

I have this problem!

Database error in vBulletin 4.2.0:

Invalid SQL:
ALTER TABLE vb_thread ADD contnum INT NOT NULL;

MySQL Error : Duplicate column name 'contnum'
Error Number : 1060

Can you help me?

P.S. Can I change language phrase?

why ?

Database error in vBulletin 4.2.0:

Invalid SQL:
ALTER TABLE vb_thread ADD contnum INT NOT NULL;

MySQL Error : Duplicate column name 'contnum'
Error Number : 1060