How is it possible to create a thesis based on the question "what impact did slave narratives have on the abolitionist movement?"
To create a thesis statement you need to take a stance, in other words to create an answer to the question and then support the answer with facts. You can take two possible stances with respect to this question. You could argue that the influence was minimal or that it was significant. In either case, you would need to do research to see the degree to which slave narratives were mentioned in the writings of members of the abolitionist movement. As you research the topic, you will refine your thesis to reflect the specific types of influence that the narratives had. For example, you could claim that they were used primarily as sentimental examples or that they were used mainly as factual support.
In general, developing a thesis is an iterative process of first creating a general thesis, then doing research, and then refining your statement on the basis of the information you discover as you research your topic.