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.

Hi Armin, which version of the Embedded Git plugin is running on your server? If you have IRIS terminal access, the following command will show the version:

zpm "list-installed git-source-control"