If a rav sees while reciting Pesukei D’zimra that zman krias Shema is about to pass, and he thinks that there are some congregants who did not say it as perhaps they don’t know that they need to say it at home before coming to shul, may he announce somehow that those who did not say it should say it now, or is that a hefsek?
Regarding your question, since saying kriyas shema is part of the needs of the davening, it may be done between paragraphs. This however is very b’dieved, and if the person can manage to get the message across without having to talk out, such as by getting the congregants attention and then putting his hands over his eyes, without talking out that is surely preferred. Alternatively he should make a sign before shabbos etc. that reads “Say Kriyas Shema now” and this way he doesn’t have to talk out in the middle.
M:B 51-10, Igros Moshe O:CH 1-22. See Eshei Yisroel 16 ftnt. 49 regarding skipping parts of pesukei d’zimra in order to say kriyas shema with the brachos on time. However for our question it isn’t going to change anything because the rabbi will surely have to announce this to the congregation, as they surely will be confused. Besides this can cause a big takala for the congregation, because it will become a habit that pesukei dzimra can be skipped.