The software repos you mean?
First to outline the problem with doing so is that there are constantly updates being issued, and whenever a newer version of a package is released the older version isn't necessarily still available. So for example it might me 30 gigs, to use an arbitrary number, to download all the software but by the time you've finished that download something might have already gone upstream... lets say for the sake of argument that it was among the last packages to be downloaded. They would no longer be there and they would be skipped or the download would fail depending upon how you did it. If they were skipped, there might be packages in the bunch that you intended to use, or worse, that other packages need to use as a dependancy. If you were to simply download the newer versions then they would have data within the package that would specify what dependancies it needed and the problem that might then arise is that a package earlier on in your download that is a dependancy of the new one also needs to be updated.
That doesn't mean by any means that it isn't doable. I've seen instructions for doing it for both a single machine using DVDs (When hard drives were smaller) and for setting up a repo mirror for an LAN. Those tutorials may be hard to find now and may or may not be outdated. The tips and tricks section of http://ubuntuforums.org is ludicrously disorganised as things from years ago that are no longer applicable, and will fail, are still there. I would still go there and look though. I'll have a look myself later in the day.
By the way I assume that you're either wanting this for an LAN or for a computer that doesn't have net access. In the case of a computer that has no net access, if you were to simply go about selecting and downloading every single package successfully without the above issue (which can actually be worked though even if you ran into trouble) then you could simply not update your sources after that (or they would then be wanting updated packages that you don't have on the drive) and put all the .deb files on their own partition, or even an external hard drive, (to keep it from slowing the box down aside from when actually installing) and mount it at the appropriate mount point, which is /var/cache/apt/archives
Its going to be a really big download.
I hope I understood what your'e wanting to do. If so, what the scenario is will be of help for anyone who's trying to help you... if its for a machine that doesn't have net access or if its to keep a repo for a larger set of computers on a network, or if its something differant entirely.