"已经向货代订舱"可以用英文表达为"I have booked with a freight forwarder."
"book"是英文中表示"预订"的常用动词，而"freight forwarder"则是指"货运代理人"。
例句：
I have already booked with a freight forwarder for the shipment.
I have arranged for a freight forwarder to handle the booking.
I have made the booking with the freight forwarder for the cargo.
中文翻译：
我已经向货运代理人预订了运输。
我已经安排货运代理人处理订舱。
我已经向货运代理人订了货物的舱位。