Yes, but first you need to call dlmread('ex.txt') which you have not done yet. You've only read in AA.txt and BB.txt.
exData = dlmread( 'ex.txt'); % Input data
Next, I don't know what the "E11" column is. Columns have only letters, not letters and numbers. Do you mean the column that cell E11 is in, which, of course, is column "E" (column 5)? If so then:
% Multiply column 5 by 10^4
exData(:, 5) = exData(:, 5) * 10000;