OK, so this topic is more "How to Install Guest Additions in VirtualBox", than "Getting Files into an OS/2 VirtualBox VM".
Once VirtualBox is install and you have installed a guest OS on VirtualBox, go to the menu bar at the top of VirtualBox, select "Devices" and the last item on the menu list will be "Insert Guest Additions CD Image", choose that option, then go to the CD/DVD drive in the virtual OS and run the installation from there.
In the dozen or so times I've installed virtual systems using VirtualBox, I've never run into an instance where the Guest Additions CD image was not included in the VirtualBox package. Never had to download or transfer files from anywhere to set up Guest Additions. That said, I've never run OS/2 virtualized or otherwise, so there may be differences I'm not aware of.
Collection = T430 - T500 - R400 - X300 - T61 (14" WXGA+) - R61 (15" SXGA+) - T60 - X40 - T43p - T43 - T42p - A30P