No, you can't, not 3rd party packages .......a "new" install (including upgrades) scrubs all existing 3rd party repositories as a safety measure.
So you will have to add or reinstall those yourself afterwards. A reason to keep a file somewhere on what and how you added yourself.
If you do "apug", the repositories will not be changed ..... as a matter of fact the extra repos are included in the update AFAIK. So if you want to keep your working machine "as is" do "apug" to keep up to date, like in a rolling release. Though that would pass over the installer testing.
Naturally 3rd party apps that are not installed through added repositories but directly through "dpkg -i" or from source will not be upgraded either. They will even be marked for removal if there are conflicting dependencies with the "official" packages.
Mmmmm another issue in 32 bit release, in HP 2Gb RAM after all day yesterday working fine and smooth, I did apug at night and 148 packets were updated.... After that I lost network connection and right panel where we see the date, hour, memory cpu and disk use is lost...
strangely in 32 bits release in the other laptop (Acer with only 1GB RAM) everything is OK even apug runs fine and no issue after apug (but last apug was Monday in Acer and yesterday in HP, so maybe there are some repository problems again since yesterday?)
When I did my first 3.7 installs I leaved Elive take all disk and partition it itself and it made a 340 MB partition that finally had 250 MB full. Since then in all installations where I changed myself partition scheme I kept always 340 MB for boot...