In Chapter 15, page 185 of the DL.3 manual it states:
"NOTE: A DL.3 fixture cannot capture SDI and output its own camera feed through SDI simultaneously. So, unlike the S-video capture, it cannot be displaying the SDI feed of another fixture while outputting its own camera through SDI."
When the SDI system is capturing, it mirrors whatever is coming on the "IN" port to the "OUT" port, which is also stated in the manual on the same page.
"While capturing SDI, anything that is being fed to the "In" SDI port will be mirrored on the "Out" port. Therefore, it is possible to daisy chain fixtures together via SDI, allowing multiple DL.3s to have the same SDI feed without a switcher/splitter device. "
So, if the signal you want to send OUT of the DL.3 is the same as what is coming into it, then yes, its possible. However, if you want to capture SDI in on a fixture and send the camera on that fixture out SDI, you can not do that.