Are there any mussar seforim learned in the Yeshivas that were originally written by a gentile and then translated into lashon kodesh, and have now become part of the Jewish mussar library. I had heard from someone that Sefer Cheshbon Ha'Nefesh was written by a non-Jew and was brought into the Jewish world by Rav Isaac Sherr.
In Benjamin Franklin's diary (see pg 38, 39), there is a discussion of making a weekly chart where each row is one of 13 virtues, and each column is a day of the week. That gives you a box for how you performed in that virtue on that day. And you make marks in that box each time you succeed or fail in making the right decision in that virtue.
This is clearly the source of the system for keeping a Cheshbon haNefesh (literally: An Accounting of the Soul) described by Rav Mendel Satanover (Menachem Mendel Lefin) in his sefer by that name. Although his list of Middos differs from Franklin's Virtues in a few of the items. A second difference is that Cheshbon haNefesh does not treat the list of Middos as canonical -- he suggests alternatives and sketches how to give them similar treatment, depending upon what the individual needs to work on. (I have a comparison of Franklin's 13 Virtues with the middos in Cheshbon haNefesh as well as the 13 middos the Torah Temimah attributes to Rav Yisrael Salanter in a blog post titled "Aspaqlaria: Lists of Middos".)
Rav Yisrael Salanter (Lipkin) was a major supporter of the book Cheshbon haNefesh. (As well as the practice.) He raised funds to have it republished. This, despite the fact that Satanover was a Maskil and a friend or associate of R Moshe Mendelsohn's -- one of the vanguard of observant German Jews who took ideas from the Enlightenment. And who are vilified in many circles of the Orthodox community because history shows us that their was the first step toward Reform Judaism. (Although as a point of history, this clique were themselves fully observant Jews.)
Another oft-quoted example is a short essay in R' Eliyahu E Dessler's Michtav meiEliyahu (edited from notes by R' Aryeh Carmell) -- vol IV pp 243, "Reviewing One’s Character; the Root of Perfection", which summarizes a talk R Dessler gave in the Ponovezh Yeshiva in Benei Beraq in September 1949. All of the ideas in the section of the essay listing practical steps one can take are found in Dale Carnegie's "How to WIn Friends and Influence People. (In fact, all are found in the Reader's Digest version, so R Dessler may have just read that.) For more, see Rabbi Yosel Catane's article, "Accept the Truth from He Who Speaks It”; Rav Eliyahu Dessler and Dale Carnegie", Hamaayan 1992 -- or R' Ari Waxman's 2005 translation on Scribd.
The key to understanding this phenomenon is to realize that Mussar's goal is to ever improve oneself in the image of G-d, to be better capable of doing His work. The tools to getting there, aside from Torah study and mitzvah observance, are not the point, nor defining features of the movement. Therefore, they were fully comfortable borrowing ideas from Franklin, Carnegie, Covey, or other self-help "gurus" and of course would borrow from the field of psychology. Adapting along the way from psychology's focus on getting rid of the issues that keep one from being who they want to be to a goal of becoming the person Hashem wants us to be.