You have several options.
One approach:
fid = fopen('YourFile.txt,'wt');
fprintf(fid,'%.6f\n',A)
fclose(fid);
Experiment to get the result you want.
EDIT —
You edited your Question, so I’m editing my Answer.
To append the column to the end of an existing file using the dlmread (link) function (consider using the readmatrix (link) function if you have R2019a or later), to read the file into a matrix of the same row size as ‘A’, or truncate ‘A’ or append NaN values to ‘A’ as appropriate to make the row sizes equal, then concatenate it to the end of your matrix. Then write it to a new or existing file name.
The concatenation would be:
NewMMatrix = [OldMatrix A];
The easiest way to write this would be to use the dlmwrite (link) function, or if you have R2019a or later, the writematrix (link) function.
