Have an AT&T plan already; need a short-term mobile hotspot and more data for travel

No, it’s not something you can add to your current plan. It’s prepaid, it would be a separate plan and account. It’s not the only plan available, prepaid plans are here:

https://www.att.com/prepaid/mobile-hotspot-tablet/

Is there any option to use my cell phone as a hotspot and purchase additional data on a month-to-month basis without changing my entire cell phone plan? 

That would depend on what plan you have but if it doesn’t already include hotspot, adding hotspot is likely not an option.

If not, can we achieve the same goal by purchasing a mobile hotspot and a one-time use sim card?

Yes. There are prepaid plans for hotspot devices. There is a 100GB plan for $55.

Thanks for the quick response.  So is that AT&T's plan for $55.  And I can purchase that as an "add-on" for 2 months to my current plan?

Thanks!

Hi forum_user_name! We are here to address your mobile hotspot and data add-on inquiry!

As sandblaster stated, you won't be able to add hotspot data on your plan that is already provisioned with mobile hotspot. We do recommend our data-only plans with both wireless and prepaid options for your hotspot device, please check our available hotspot devices that can be used on our data-only plans. Also, learn how to set up a data-only plan for your tablet, computer, mobile hotspot and other devices. 

Lastly, check out our AT&T Prepaid hotspot plans. This won't be an add-on your cellular device but a prepaid data only plan for your tablet or hotspot device. 

We hope this helps! Reach back out to us if you have additional questions. Thank you for reaching out to the AT&T Community Forums!

Rhoda, AT&T Community Specialist