I haven't tried it, but having a look at the original m0n0wall files it seems like we need to import all of these files:
captiveportal.db
captiveportal_mac.db
cpelements
dhcpd.leases
dhcpd.leases~
routes.db
voucher_active_2.db
voucher_active_3.db
voucher_active_4.db
voucher_active_5.db
voucher_active_6.db
voucher_used_2.db
voucher_used_3.db
voucher_used_4.db
voucher_used_5.db
voucher_used_6.db
Is that correct, jstrebel?
captiveportal.db
captiveportal_mac.db
cpelements
dhcpd.leases
dhcpd.leases~
routes.db
voucher_active_2.db
voucher_active_3.db
voucher_active_4.db
voucher_active_5.db
voucher_active_6.db
voucher_used_2.db
voucher_used_3.db
voucher_used_4.db
voucher_used_5.db
voucher_used_6.db
Is that correct, jstrebel?