No possible except via DRM stripping, I am afraid.
What device are you using Mobi on?
If it's a tablet, there are quite a few eReader apps for Android and iPad that will take Adobe DRM (Bluefire, Aldiko (not iPad), Overdrive).
Might be worth checking them out.
Ahh poop. Too bad
reading on PCs (desktop and small laptop)
You could also try Sony Reader. That is designed to go with Sony eReader devices but is a free download that can work standalone, and can read Adobe DRM books.
I don't like it very much, but you may happen to prefer it to ADE.
You might also try the older ADE. That gives a better reading experience to my mind than the new one, as well as having many fewer bugs.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
For non-DRM books, try Calibre. http://calibre-ebook.com/
If they aren't DRMd/secured mobi ebooks, Calibre will do the changeover, BUT... I do this for a living (MAKE ebooks and print books for publishers and self-publishers), and going from ePub to Mobi with Calibre will always give you better results than going the other direction. Not that you have that option, I understand. In fact, my process is to always make the ePub first and move from there to Mobi/PRC to get best results in end files. No offense, but if you want to convert something that is already in it's final Mobi state to something else, you're never going to come up with the kind of results I get making a file from scratch and going through the logical steps from one format to the epub or mobi books. I used Vibosoft mobi to epub converter: this is the guide how to convert mobi file to epub format.
or Coolmsuter app:
all of them works,but it is paid!
You can have a try iStonsoft MOBI to ePub Converter for Mac to Convert MOBI to ePub on Mac,Follow Step-by-step Guide:
Step 1: Import MOBI books
Step 2: Output folder
Step 3: Start conversion
Yeah, i agree with the solutions above, and i'd like to share my way with you here! If you want to transfer your ePub format to MOBI, you can either try Calibre or iPubsoft ePub to MOBI Converter! And if you want to realise the opposite function, view this guide here which tells you how to convert MOBI files to ePub, which i think is also very useful: