Hi,
As Gaurav suggested we have to give the location of the xml file all the way to the file itself. For example, /usr/sap/SID/SUM/xxxx.xml. What is easy is to place the xml file in the same location as the stack so that the SUM tool can pick the stack from there.
What we do is, place the stack in a central location and mount that location to the server being upgraded so we can use the same stack for all the systems in the landscape.
Also make sure of the file permissions for the stack.xml file as it might still have the same permissions of the solman system.
Thanks,
Asad.