MATLAB will recognize such a card as being a serial port if the OS considers it to be a serial port the first time in the MATLAB session that you ask the OS for information about (or to connect to) the serial ports. If the device comes with a proper serial port driver (nothing special, just the regular driver) then it should be usable with MATLAB.
ExpressCard is "hot-pluggable", so if the card is not plugged in when you start MATLAB and first request serial operations, then you will have to restart MATLAB to make the card visible. MATLAB does not currently support any mechanism to re-ask the operating system what ports are available.
ExpressCard ports (that MATLAB knows about) should be as usable as hardwired serial ports or serial port cards.
In your situation, many people would tend to use a USB to Serial convertor. That is supported by MATLAB as well (again with the limitation that the device has to be connected when you first do a serial operation). USB to Serial convertors, though, have a minimum 40 ms latency (this is part of the USB standard; MATLAB does not provide support for the USB mechanism to reduce the delay), so USB<->Serial is quite restricted in the number of serial interrupts per second. To the best of my current knowledge, the latency for the ExpressCard solution should be much lower and the number of supported interrupts per second should be much higher, making the ExpressCard solution more usable.