You can install the DLC manually to a hard drive, but I don't know about installing it manually to a USB drive, as I don't have that feature yet. But just in case it helps, here's some info.
For a manual HDD install, you'd need to grab the DLC files in the DLC disc's folder content/0000000000000000/FFED2000/FFFFFFFF/, then copy them to content/0000000000000000/425307D5/00000002 on the HDD.
You'd also need manually install the title update into the cache. I don't know how to get that off the disc, I just found it on a website somewhere, then copied it to the cache directory. If you're still using Live on this system, just playing FO3 should get you the title update (NOTE: I have no idea if installing this DLC manually will get you banned or not, if you're worried about it, don't do it.)
Good luck, I don't know if any of that helps you or not. Hopefully USB is similar enough that you can figure it out or someone else can point you in the right direction for the rest of what you need to do.