The prefix Z8 was assigned to South Sudan by the ITU. To make the changes in your log and in the 'dxcc.rsd' database you have to perform the following steps.
To convert ST0 to Z8:
Quit 'RUMLog' and start 'EditDXCC'.
Backup your log file the the 'dxcc.rsd' database file
In 'EditDXCC' select Menu-->DXCC-->Change DXCC Prefix
Select your currently used prefix for South Sudan from the popup menu
Enter 'Z8' in the right hand text field
Press the 'Run' button
Now we can add 'Z8' to the prefix list:
Select the "Prefix" tab
Press the "Add Row" button
Enter the data as shown below
Mouse click the Prefix header row to sort the table by the prefix - for your convenience only
Press the "Save' button
Quit 'EditDXCC'
Now you can start 'RUMlog' and all South Sudan QSOs show up as 'Z8', no further action is required.