Note that if spaces are needed then quotation marks are needed at definition and must be chopped while concatenating:

rem The retail files set 

set FILES_SET="(*.exe *.dll" 

 

rem The debug extras files set 

set DEBUG_EXTRA=" *.pdb" 

 

rem Build the DEBUG set without any 

set FILES_SET=%FILES_SET:~1,-1%%DEBUG_EXTRA:~1,-1% 

 

rem Append the cloasing bracket 

set FILES_SET=%FILES_SET%) 

 

echo %FILES_SET% 

stackoverflow.com/questions/856573/concatenate-variables