Import your boxbilling clients to whmcs database with single script.
01). Add IP to WHMCS API IP Address list.
02). Add your Boxbilling Database details to import script.
03). Add your WHMCS API information to import script.
04). extend your PHP execution time to 1800 or 3600
05). run the script.
06). then run the activate.php to activate client accounts.
ta da all your clients in boxbilling now copied to WHMCS.