Can I use an iMac as monitor for MacBook Pro?
No, as the first iMac which allows this is the 27-inch Late 2009 iMac, as explained here.

There are, though, some Applications which may achieve something similar, requiring both computers to be connected to the same LAN/WiFi/VPN, and sending all the information, the one to be displayed on the second computer working as a display, through it. Such as ScreenRecycler. You can find some more here.