If you want to add two lists, use List_Concat, use List_Add to append a single value to a list.
Basically everything available is at the wiki link I gave one post above.
You could also do it "manually", but unless something is impossible to do with only the lego scripts (which I find quite unlikely), I'd advise against doing so.
There's also List_InsertSorted(list, value, comparator) which is not in the official documentation yet (apparently...), I guess I'll add it after this post. Could be useful for your messaging system (maintain a list of messages that is ordered by their lifetime? Unless they all live for the same time...).