This is not the answer you'll expect, but I'd say: do not even try it. If you import a PPT, which is non-responsive, into Ca^ptivate it will be a normal project. Each slide is converted in a movie slide, keeping all the animations from the PPT. There is no easy way to convert a normal project into a responsive project. Since you don't have the separate objects from the PPT, it is practically impossible to set up size and location in different breakpoint views. You can only rescale the 'movie slide's', I don't think that result would look fine on a small mobile browser screen.
Use the assets, the storyboard, the backgrounds from the PPT but create a responsive project from scratch in Captivate.