[Koha] Importing records preserving the system number

I'm trying to migrate about 140,000 records. Each of them has 001 with the original value. I copied it into 999c and 999d. Importing in Koha, 999c and 999d are replaced by a new (autoincremented) bibliotemnumber. How can I force Koha to assign bibliotemnumbers using 001 or 999c/d? Thank you. SB