To see how to install a USB hub, GPS, Bluetooth, a card reader, flash drive, high speed WiFi, a modem and other add-ons to your ASUS Eee PC see "Eee PC Internal Upgrades". I have not tried any of these. As well as voiding your warranty these could destroy the computer if not done correctly. But many involve just adding USB devices, so are not that hard.
What would make this easier would be an external docking device which would fit under the computer and have USB sockets to hold USB drives. This would stick on under the computer and plug in with one USB cable. It would then supply four recessed, widely spaced USB sockets designed to hold Flash, Bluetooth, WiMax, 3G, WiFi, Ethernet and similar devices.
The one docking unit could be designed to fit the Eee PC and the Apple MacBook Air (which is lacking in sockets and expansion). The unit could optionally hold an extra batter battery to power the devices, recharged via the USB cable.
Of course given the relative costs of Eee PCs and Apple Airs, you could afford to use an Eee PC as a docking device for an Air. ;-)
ps: I suggested building "flash docks" into a low cost education computer. The idea came from some of the early subnotebook computers which had lots of PCMCIA slots and very little else. To add memory, a disk drive or any interface, you added a PCMCIA Card. Now many such expansion options are available with USB, but some way to physically hold the USB device is needed. The ExpressCard is intended as a PC Card replacement and includes USB as an option. But Express Cards are expensive and rare, compared to USB devices.