Way to add steps without compatible device?

There is no way to manually add steps that then give you an estimate for Calories. You can note the number of steps in the notes section of your Exercise Diary. Or, you could create a custom measurement using the web version which will then sync with the mobile app, and enter number of steps using the created custom measurement in the Check-in (web)/Progress (mobile app) area.

​Please see these articles, and search for answers to Frequently Asked Questions and other helpful articles, on the MFP Help/FAQ pages...

https://myfitnesspal.desk.com/customer/en/portal/articles/1027586-how-do-i-add-notes-to-my-exercise-diary-

https://myfitnesspal.desk.com/customer/en/portal/articles/10704-can-i-track-additional-measurements-or-change-the-default-measurements-