First of all, let's realize that not every single bit of text you have students read is in a textbook and untouchable.
Second, consider seeking out appropriate content sources, such as newspapers, that students can indeed learn this strategy with while still pursuing meaningful social studies goals.
Third, think about how you can get around the problem of textbooks that can't be marked in. For instance, in order to teach the strategy, you might photocopy a page or two out of the text that students use and distribute it to them. Make an overhead of that selection for yourself. Model for them and guide them in practicing the strategy on the photocopies. Alternatively, if you have enough of the materials available to you, give each student a sheet of transparency film, some paperclips, and some overhead pens. Let them practice directly on their texts by using the transparencies.