Is there a way to use multiple SD-memory cards with the Raspberry Pi, perhaps in RAID 0 configuration? It would be a good way to expand the available memory, reuse some of extra the cards that I have, and keep the Wattage low (compared to an external drive).
there's only one SD slot on the Raspberry Pi, so you'll need some external hardware, and as far as external hardware goes, you cannot beat a good HDD with a pack of old and faulty flash cards.
and regarding the power consumption, contemporary 2.5" HDD consumes about 0.15-0.2A on average, that's about 1/4-1/5 of what your Pi consumes, you can hardly go any lower than that using powered USB hubs and all this stuff you'll need to access your SD reader cards.
In relevance to this Super User answer.
You can get a whole lot of SD to USB adapters because there are no multi SD port readers. Then you set up raid in Linux or use ZFS on the SD Cards
4 Ports SD to SATA Adapter
Runs in RAID-0
You will need to use a SATA to USB to use this.
Runs in RAID-0
You can get a CF to USB convertor.
1. solution: USB style
Is your attention only to extend program memory? Then, as @Gerben already said, buy/use USB SD-card reader. But than you don't need RAID 0 configuration. It is meant to be for improving performance while using data striping, but major drawback is fault tolerance: if one element in array dies, whole array is corrupted.
2. solution: network style
One solution could be to search for any (free) hosting, like ftp, http, etc. Then just copy user data to web storage. Or use your own server, like wamp. You can use ethernet or USB (e.g. USB WiFi dongle, USB Bluetooth, etc.).
3. solution: GPIO style
There are also some GPIO pins already on the board with various protocols: UART TTL, SPI, IIC. Those can be directly connected to other external devices via TTL pins.
So, the only limitation is your imagination ;)