Converting Firewire to USB 3 - is it theoretically possible?

Short answer: No.

Slightly longer answer: Yes, but you'll essentially need another computer in a box to do it.

USB (1.1, 2.0, and 3.0) are based on completely different specs than Firewire (400 and 800). Even the way they interact with the processor and memory is completely different. They are not wire-compatible, or even signal-compatible. In order to convert a a Firewire connection into a USB connection, you would need a computer with both ports, have it convert the Firewire signal into binary, then convert the binary into a USB signal and send it out. This hasn't even touched on the problem of having the device show up properly to the end computer. It's a complex problem that hasn't be tackled because there is so little demand for it, especially considering the price that would be asked for such a device.

If you use Expresscard solely for Firewire, I say ditch Expresscard and find something with built in Firewire. It's growing more uncommon for it to be included on consumer laptops, but it's still widely available on business models, which also tend to be significantly higher in quality.

For the purpose of adding a FireWire port to a host: Thunderbolt is the successor to ExpressCard (not USB 3). Devices that have historically been FireWire will likely split into USB 3 on the low end and Thunderbolt on the high end.

Thunderbolt is available on non-Apple laptops in addition to current Apple hardware (of course running Windows on it makes the adapter moot as the MacBook Pro also has FireWire).

As far as I know Apple's $29USD Thunderbolt -> Firewire adapter is the only one shipping. With the main caveat that its bus-power is limited to 7W. Both the OSX86 folks (Mac OS X on non-Apple hardware) and the Bootcamp folks (Windows on Apple hardware) seem to be using it effectively.

If you want to actually replace the ExpressCard functionality itself you can get something like Sonnet's Thunderbolt -> Express Card adapter which of course can host an ExpressCard -> FireWire card.

In theory one of the announced docks will ship eventually.

• Belkin Dock: 1 Gigabit Ethernet port, 1 FireWire 800 port, 1 Thunderbolt port, 1 3.5mm Headphone Output Jack, 1 3.5mm Audio Input Jack, 3 USB 3.0 ports
• Sonnet Dock: Thunderbolt, USB 3.0, SATA and eSATA, FireWire 800, and Gigabit Ethernet, and an internal 2.5"/3.5" drive bay and an optical drive bay.