I know you can backup a copy of the ADE key from the Windows Registry by doing the following:
- Close all applications.
- Choose Start > Run.
- In the Open text box, type regedit and then press Enter. The Registry Editor opens.
- In the left pane of the Registry Editor, locate the following registry key: HKEY_CURRENT_USER\Software\Adobe\Adept
- Select the Adept key.
- Choose File > Export.
- In the Export Registry File dialog box, select Selected Branch under Export Range. Enter a name and location for the backup registry key, and then click Save.
The problem is getting the key into a form Calibre can use. I was in a similar boat just a bit ago and ended up having to install Calibre, this plugin and DeDRM in the VM along with ADE. From there, I imported the key in both this plugin and in DeDRM and exported the key (as a .DER, I believe) from each plugin for safekeeping. That key is what I imported into plugins on my Linux version of Calibre.
I haven't run across any way to convert that ADE Registry key to a usable .DER file. The only use I've had for it has been re-importing it back into a new computer's Registry so I didn't use up my activations.