Here is how you do string - list:
seq(Instring("ABCD...",sub(str1,A,1)),A,1,length(str1)-> L1
EDIT:
Here is a way to convert the list to string:
ClrHome
" /->/ Str2
"ABCDEFGHIJKLMNOPQRSTUVWXYZ? ./->/Str1
For(A,1,dim(L1
Str2 + sub(Str1,L1(A),1 /->/Str2
End