To make your own stamp Category and create Custom Dynamic Stamps, please follow below instructions.
1. Go to the Comments Tab, click on Stamps to select Custom Stamp->Create Custom Dynamic Stamp.
2. Select a Template or click on Create Template... button to Browse one for your Own.
3. Add a Stamp Category and Customize Your Stamp.
Finally, the created Dynamic Stamp is shown under Stamps.
How to Import Dynamic Stamps from Acrobat Reader to Foxit PDF Editor for Mac