It is possible and I have done it. When doing the virtual box install select linux, and set the distro to other linux. Are you going to be using a CD, flash drive, or virtual mount?
Slackware sessions have been relatively simple in virtualbox for some time, however the slackware-current 32-bit version since the release of 13.37 has had issues because HIGHMEM is installed which messes with virtualbox. You will need to setup the vbox session to enable PAE for it to boot.
OMG, after a very long time, got some replies. Anyway, now Slackware is my main Distro . But, when I tried to install in virtual box in 2011, each time it was trying to install the distro, never boot into it.