You can't add RAM to a raspberry pi but raspberry pi clusters are another thing
Check : http://www.zdnet.com/article/build-your-own-supercomputer-out-of-raspberry-pi-boards/
UPDATE:
Since someone requested, I will summarize the link above. Although I myself is not familiar with the details, it seems that it is possible to "connect" raspberry Pi(s).
"Kiepert ran the High Performance Linpack (HPL), the standard supercomputer benchmark on his home-made computer and found that his RPiCluster with its 32 Broadcom BCM2708 ARM11 processors running are 1GHz and 14.6GB of usable RAM turned in a HPL peak performance of 10.13 GFLOPS"