Yes, you can certainly ADD more stuff very easily, yes.
As for removing stuff or adding stuff, you can do it from the command line too, with the command, for example:
The first example would remove Gaim and everything that was installed with it.
The second one would install a slackware-packaged (.tgz) archive.
And to run the entire setup version of the installer, just type pkgtool at a command line, and the graphical (textual really) installer will pop up in the console.
It really is a very efficient package managing system.
If you want to use pkgtool to install stuff from a disc, or a folder somewhere, just havigate to the disc or folder, type pkgtool at a command line, and presto!
Also, lets say you go "remove-crazy" and remove too much stuff, or remove something critical.. Then you boot your CD, run the installer again, pick your destination partition, and re-select the stuff you need for reinstallation, and presto. Fixed