Can the mitosis of cancer cells be stopped by inserting the appropriate signal molecule into cells?

Hopefully. This is one of the ideas behind targeted therapy. One of the first successful approaches has been in chronic myelogenous leukemia where the abnormal protein that causes cell division at an enhanced rate is blocked by a designer molecule. This has resulted in long term remissions and preference for this approach over the prior gold standard of allogeneic bone marrow transplantation.
Possibly. Possibly...And researchers are constantly seeking the answers to questions such as yours. Hopefully, one day I will be able to answer you with an overwhelming yes.