You can. It's slightly more involved than the Kindle DRM stripping, but it's not bad. I was able to do it and I don't even own a Kobo. I found a book I wanted that was for sale on the Kobo store but not offered by Amazon.
For those who want to try this at home, you're looking for the Calibre Obok Plugin. I think it's usually part of the DeDRM package which you probably already have. I can't remember where I found instructions but this page seems more or less accurate:
There is one simpler way of accomplishing the same thing: just go to LibGen and look for the book you bought. And download a fresh copy from there. That might be the best solution.
Okay, relevant to the discussions of e-reader formats: Why, on my 2014 kindle, do MOBI files converted from ePub not support all the fonts on my Kindle? A native purchase, or even at times a non-native MOBI direct, looks better than an epub->mobi conversion. I had to create a new preset on the Kindle for font and margin to make it look tolerable.
Good question. I don't know the answer. I think the 2014s supported AZW3 so you might want to try that as your output format from Calibre. I say that because I've never seen any unavailable fonts in an epub > azw3 conversion.
For those who want to try this at home, you're looking for the Calibre Obok Plugin. I think it's usually part of the DeDRM package which you probably already have. I can't remember where I found instructions but this page seems more or less accurate:
https://www.epubor.com/calibre-kobo-drm-removal-plugin-obokp...
There is one simpler way of accomplishing the same thing: just go to LibGen and look for the book you bought. And download a fresh copy from there. That might be the best solution.