Can I have multiple wlan cards on one PC and connect to multiple connections?
Yes absolutely, but it probably won't be as useful, or as easy to setup as you are hoping it will be.
Is it possible connect to 2 or more wlan connection and use the internet with improved speed?
It depends on the Operating System. On Windows/Mac OSX this is nearly impossible. Under Linux it is only very difficult.
For example i have one 2Mb/Sec Internet and one 5Mb/sec internet connection. Is it possible that I can connect to both at a time and use at 7Mb/secc?
Any possible solution is going to load balance the connections, so if you are downloading 2 things, and where connected, each download might use a separate connection. But if you are only downloading a single file it will only cross one of the links.