As Gareth said, it is possible but takes a bit of work. I've done it with other themes and with Adaptable, but Adaptable took more work. You have two options:
1) Make a child theme. This is fairly easy to do - search Moodle documentation for instructions. The disadvantage is that you will not have the graphic interface for configuring the child. You will have to make all your changes in CSS files.
2) Create a clone. This will involve changing some file names and quite a few lines of code. I use editing software such as Notepad++, Text Crawler or FAR in which I can search/replace a directory and files for all instances and variations of instances of some text. So, for Adaptable you might need to do the following search/replace in all files and filenames (assume your cloned theme is NewAdaptable), upper/lowercase is very important:
a. Adaptable replace with NewAdaptable
b. adaptable replace with newadaptable
c. _ADAPTABLE replace with _NEWADAPTABLE
d. _adaptable replace with _newadaptable
e. is_desktop replace with is_newdesktop (I ran into some problems with the following function names on my first try)
f. is_tablet replace with is_newtablet
g. is_ipad replace with is_newipad
h. is_mobile replace with is_newmobile
A word of caution - try this on a test server first. The first time I tried cloning Adaptable I did not get all the changes made correctly, and it threw a bunch of errors and prevented logins. I had to SSH into my server and remove the cloned theme.