To save an Impress presentation as a powerpoint .ppt file you will need to go to File > Save As from the menu bar on Impress. Then select the location where you want to save the .ppt presentation.
Make sure to choose Microsoft powerpoint 97/2000/XP (.ppt) in the File Type combo.
When you are done, Click Save. The message shown below will appear. Click Yes to confirm that you want to save the file in Microsoft powerpoint format. You can select the checkbox to not show the message again.
It is recommended to save your work in Impress OpenDocument Format (.odp) before you save the .ppt because the compatibility may not be 100% so you are under the risk of losing some formatting or animations.