First, you just need to open Keynote and then specify the .ppt file to open the presentation in powerpoint as the original format, this can be done clicking in “Open an Existing File” at the bottom of Theme Chooser. Now, you will have your presentation opened in Keynote. In order to save the presentation in Keynote format, you will need to Save As the presentation and specify Keynote instead of powerpoint.
It is important to be aware that converting from powerpoint to Keynote may generate some inconsistencies and differences compared with the original presentation, this is mainly caused because powerpoint and Microsoft Office is a much more complex and mature software.
Convert powerpoint to Keynote in batch mode
You may need to convert your powerpoint presentations to Keynote in bulk mode, or many PPT files at once. There are ways to achieve this task by using automation. With Automator in Mac you can convert powerpoint to Keynote easily with a simple batch process. You can learn more in this Macworld article.
If you need to make prototypes, you may consider to review the prototyping and wireframes PPT templates available in Keynotopia.