The early releases of Newsbin 5 did not support this capability. Fortunately, NewsBin v5.05 and later does support this.
However it is still recommended that you use Auto Headers. This will allow NewsBin to continually download headers in the background so that when you want to load a group or folder, it will already be up to date. This will also minimise the use of valuable RAM while downloading headers.
Since v5.0 remembers which posts are new, and only clears that status when told, you can download headers as many times as you want without having to worry about missing any new posts.
On a cautionary note, releases prior to 5.10 may miss displaying a small fraction of headers while they are being downloaded. Quade has managed to replicate this, and is working on a fix. If you suspect that this has happened then try a reload will ensure you see them all.