You have 5 USB ports already, but yes you can purchase a pci usb card and install it in one of the pci slots if you wish.  Don't know much about the second question, but have you considered either a USB mouse and keyboard or even a Wireless one  ?  They really don't cost all that much and so much better than the older connectors

Several companies make USB to PS/2 adapter cables, some with 2 PS/2 connectors into one USB port.  I've never seen an adapter card to add the ports.  Do a google search on "USB to PS/2 adapter".

Dave

Why not a simple USB hub with multiple ports plugged into one of the USB ports, or am I missing something here?

I added the optional PS2/serial card that provides 2 PS2 connections when I ordered my XPS400 from Dell. I didn't want to give up my older style keyboard that I've been using for years. I also needed the serial port for my UPS. It should be available from Dell as an upgrade.