Hi there! The answer is all three! You can add a pass either in the app, on the website, or you can call in to 611.
From the app, click the 3 lines on the left upper side of the screen to open the menu, then click Usage and Plans, click "View Line Details" on the phone number in which you wish to place the pass, then select "Data Passes" from the next page. Scroll down to "One time data pass" and select the one that you want.
And yes! You can use your data pass for mobile hotspot!
Thanks! Just what I needed to know...