You should be able to, the expansion card should act as an interface. You'll need to install the Soundcraft drivers on your laptop. I don't know the channel limit for Firewire though, it's pretty high but maybe not as high as MADI--they don't seem show the multi-dig card capacities in their brochure on I/O. It should be at least the 24 channels you were getting with the HD24.
From what I've read the firewire limit is 32 channels. MADI is 56 or 64. That brochure they show I don't think is really for the Expression series. They do list the card further down on the site.
MADI is different, it's just a transmission protocol, and I don't think there's ever been a laptop with onboard MADI support. You'd need an external interface for that. RME has an express card one, but it's not real cheap.
I've seen the external MADI laptop interface, $1500-$1700. OUCH.
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Another option is three of the multi-digital cards and then ADAT to your HD24. That is more expensive but arguably more robust than Firewire, and wouldn't require any driver/software install.
Since there are two consoles I'll be recording from, I don't think buying 6 cards at $400 each (maybe less from a dealer, rather than on-line retailer but..) is economical.
I'll have to buy a laptop but I think the expansion cards will be venue expense.
One advantage of a digital mixer is that you should be able to get your own submixes routed to the digital outs, or even direct outs to the digital outs. That should be ideal if the channel gain is set correctedly; you aren't affected by the main or monitor mixes at all, and if you're really clever you can save your settings (presuming the channel assignments tend to be the same).
I'm hoping for direct outs, I don't care much for groups. I like having complete control over each channel. Most channel assignments will likely remain fairly static but since this is a new adventure for the venue, I'm not real sure of the FOH workflow just yet.