Depends on which scripting language you are using, VBScript or JavaScript.
In VBScript you use
CODE
var = var & "new value"
and in JavaScript you use
CODE
var = var + "new value"
If you don't know which you are using it will most likely be VBScript.
People seem to forget, ASP isn't a language its a framework so when trying to find out how to do some sort of basic language operation like that you need to look for the "VBScript string concatination" not "ASP string concatination".