Hi Diejeitor123,
No you can not change this port but since version 16.0 there is another way ...
I assume that you forward the port 80 for the nas1. So in the main setup menu, you must enable the proxy gateway (be careful to read the help as in this case you must change all default password for the media servers from nas1) and put in "redirect NAS" field the string "nas2name;nas3name" where nas2name and nas3name are the name of the two other nas.
Then to connect to nas2 or nas3 use the url: http(s)://yourDDNS/nas2name or http(s)://yourDDNS/nas3name
Other solution, on your router you can cross the port. For example: outside port 82 and inside port 80 toward the local IP of NAS2 then the url is
http://yourDDNS:82