Dear Taik,
I suppose you have not got the meaning of S-parameters completely. When you terminate the output port of s 2-port network with a load you will end with a 1-port network. If the load does not changes the 2-port parametrs (I mean it does not load your previous network) you will be able to derive the S11 of the resulting 1-port network with simple mathematics.
But for my question, if I am given the S-Parameter of a 2-Port network and now a load of R is placed at port 2, how do I find the resulting s-parameter matrix?