Will this app really can expand an android phone?
No piece of software can increase the amount of hardware (which is what RAM is), that your phone has. If you have 512MB of RAM, then you will always have that much. What this app does (as mentioned in the above comments) is create a swap file, similar to a Windows pagefile, which can act as RAM when your normal RAM gets near full. There are a couple issues with this:
1) your internal memory was not meant to undergo the many read/write cycles that RAM must go through. This could cause rapid wear on your internal memory.
2) your internal memory has a much slower read/write than RAM, so any data stored in it will not be accessible as fast.
3) An app like this could wreak all sorts of havoc on your device if it tries to create a swap file larger than the available memory in your device (i.e. if you set the swap file size and then try to use more of your internal memory.).
All in all, the marginal speed in create you might receive from an app like this is most likely not worth the risks to your device.