こんな感じか
if (document.selection.Text == "")
document.selection.SelectAll();
var map = new Array();
map["hoge"]= "foo";
map["piyo"]= "bar";
var s1 = document.selection.Text;
for (var i in map) {
s1 = s1.split(i).join(map[i]);
}
document.selection.Text = s1;