If I bought bread that is not Pas Yisroel, is there a way to make it Pas Yisroel?
Yes. Shulchan Aruch (Yoreh De’ah 112:12) writes that if a non-Jew baked bread, it can become Pas Yisroel if a Jew is mashbiach(improves) the bread by placing the bread in the oven and continuing the baking process. The improvement is effective only if it is the result of additional baking. For example, if bread had a light color and a thin crust, and re-baking the bread will make it darker and give it a thicker crust, this improvement will render the bread Pas Yisroel. Similarly, Rabbi Belsky, zt”l held that if a Jew toasts bread or bagels to enhance the taste, this will effect a Pas Yisrael status. This is also the position of Rav Sternbuch (Teshuvos V’hanhagos I: 444).
In contrast, Sefer Issur V’heter Ha’Aruch(43:11) writes that heating cold bread to make it fresh does not make the bread Pas Yisroel since there is no change in the degree of baking