How to transfer signed tx from coldstorage to online system for submission into blockchain? I am building an exchange for crypto currency. I need to be able to transfer tx from cold storage(offline) to hot wallet(online system) for submission. Usb stick is suitable for an individual user mining but is not suitable in an exchange environment.   

One option I am proposed is to build an  api that sends tx over ethernet cable to and from cold/hot storage. But in this case the cold storage wont really be offline, it would still be connected. So what other options do I have that could work well for an exchange?
I would reroute all incoming deposits to cold storage, which I would keep in small portions on different addresses. 

Then I'd import some cold storage addresses to my hot wallet whenever my hot wallet is running low, keeping the amount in my hot wallet in the range between "not empty" and "what I'm comfortable having in the hot wallet".

As Mike mentioned, multi-signature addresses could provide additional security, and accountability.