OFC it's possible to LAN SAMP.
First, decide which PC will have the server (usually the faster one).
Then, start SA:MP on that computer, add a new server to favourites with the IP 127.0.0.1.
Now, click start->run, then type cmd in the box.
A black box will pop up, type ipconfig in there, and it will give you a bunch of information. Find the one that has the same name as your LAN connection, and under that it will show you your IP address. Write this down.
Now, on the other PC, add server to favourites with the IP that you wrote down.
Connect and play!