And USB 3 devices "Should" work with a USB 2.0 cable on usb 2.0 ports as well.
I do use USB 2 devices on USB 3 ports without issue
There is no converter because USB 3.0 is FASTER and provides more power. You can not get this extra speed out of 2.0(they even use a difference signaling method) and the extra power is not there either.
I would not worry, it will be a while before USB3 is mainstream and there are not many devices yet. Also a USB 3.0 PCI-E card is not too expensive if you need it on your current system in the near future(as long as you have a X1 slot accessible)