You might be able to use the Size property to keep track of the initial size of the stream, write some more data, then use MoveTo() to rewind back to where you started writing:

set stream = ##class(%Stream.FileCharacter).%New()
do stream.Write("hello world!")
set tmpSize = stream.Size
do stream.Write("goodbye world!")
do stream.MoveTo(tmpSize + 1)
write stream.Read()

Try the following:

set mylist = ""
for i = 1:1:5 {
    set item = "item"_i
    set mylist = mylist _ $ListBuild(item)
}

zw mylist

$Listbuild lists have a separator that includes the length of the data in the next item of the list. This is why your original code wasn't working - $listbuild(mylist, item) would make a new list with two elements where the first item was the original list.