You can save the original personal message first in a variable, like this:
code:
function OnEvent_Signin() {
var originalPSM = Messenger.MyPersonalMessage;
}
Also, I recommend to use a 
switch case block instead of using every time 
if elseif. It makes the code easier to read. 

Here's a simplified version for your script:
code:
function OnEvent_MyStatusChange(Stat) {
switch(Stat) {
    //Appear Offline
    case 2:
    Messenger.MyPersonalMessage = "I'm appearing offline - " + originalPSM;
    break;
    //Online
    case 3:
    Messenger.MyPersonalMessage = "Online - " + originalPSM;
    break;
    //Busy
    case 4:
    Messenger.MyPersonalMessage = "I'm busy - " + originalPSM;
    break;
    //Be Right Back
    case 5:
    Messenger.MyPersonalMessage = "I will be right back - " + originalPSM;
    break;
    //Idle
    case 6:
    Messenger.MyPersonalMessage = "Idle - " + originalPSM;
    break;
    //Away
    case 7:
    Messenger.MyPersonalMessage = "I'm away - " + originalPSM;
    break;
    //In a call
    case 8:
    Messenger.MyPersonalMessage = "Shh! I'm in a call - " + originalPSM;
    break;
    //In a call
    case 9:
    Messenger.MyPersonalMessage = "Mmm, I'm eating! - " + originalPSM;
    break;
}
As you can see, every 
case block ends in a 
break statement. This is because if you don't, it will execute all the other code too! 
I hope that helps... 
