Luther admits adding the word alone to Romans 3:28 saying:
“I know very well that in Romans 3 the word solum is not in the Greek or Latin text — the papists did not have to teach me that. It is fact that the letters s-o-l-a are not there.....Because Dr. Martin Luther will have it so!".... (Translated from "Ein sendbrief D. M. Luthers. Von Dolmetzschen und Fürbit der heiligenn" in Dr. Martin Luthers Werke, (Weimar: Hermann Boehlaus Nachfolger, 1909), Band 30, Teil II, pp. 632-646. Revised and annotated by Michael D. Marlowe, June 2003.)
To me that offends in two ways, first off, it contradicts Scripture which says:
James 2:24 Ye see then how that by works a man is justified, and not by faith only.
And we know that Luther knowingly did so, unless you want to say that he did not actually call the Epistle of St. James an "epistle of straw."
Secondly, he admittedly adds a word to Scripture which isn't there and thus violates the rule often quoted by Protestants:
Rev 22:18For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
Now, although he justifies his action by claiming that this is the gist of St. Paul's meaning, he must be wrong for St. Paul said previously, Romans 2:13 For not the hearers of the law are just before God, but the doers of the law shall be justified.
Therefore St. Paul says that unless one works and keeps the Law, one will not be justified by God. Therefore faith "alone" can not be what St. Paul meant. Because the idea directly opposes Scripture in James 2 and Romans 2.