Imaging from USB without network access

I've had a bit of a look around - may have missed a keyword in my search.

I have a bootable El Capitan USB with casper imaging on it which is great, but instead of it pointing to the JSS to pull down all the info for imaging I want it to read the DMG / Packages straight from the USB (to save network traffic and time).

Is this a possibility? How would I go about it?

Thanks!