Yes, you can Fold Offline. The easiest way is to run the Client with a shell like DFGUI, found here
http://gilchrist.ca/jeff/dfGUI/
It lets you set it as to be run Offline in the Config Menu, and then Stop it and Upload when online. I am not good at explaining so I will leave that up to my Peers It does have good documentation, but someone should point you to extra help that is dead easy to read .