There is a menu namely Add To Reading List, however most users don't use it much. And if you user non-Safari browser and still want to use Reading List, just follow the steps below. What you need to do is creating the new Service OS X using Automator.
Instructions to add Safari Web Page to reading list on anothre web broser
Step 1. Run Automator inside the app folder.
Step 2. Then choose the type of service document.
Step 5. Now move Run AppleScript to the right side.
Step 6. Erase the existing code, change it with this script add2readinglist.scpt.
Step 7. Save the created Service.
Step 8. The Service will appear on the menu Aplication Name > Service. The name of it depends on what you saved on the previous step.
To start sending a webs= page from another browser or app, simply select the url, then head to Application > Services. Then the URL will be automatically added to the Safari's Reading List.