After trying several hours and links, I think, I've finally got the right solution, that allow me to open a .themepack file with file-roller.

1. First install cabextract package, by entering the command on terminal.
   
   
   
   
   • Press super+a key on Unity, type terminal on dash and click on the terminal icon.
   
   
   • Type sudo apt-get install cabextract command on the terminal, provide your password and hit enter.
   
   



2. Then type again in the terminal sudo apt-get install assogiate, this will install the assogiate program on computer. (alternatively, you can install it through "Software Center".




3. Then open the assogiate program, by pressing alt+f2 and typing assogiate on the 'Run a command' box.

   
   
   
   
   
   • This will open assogiate program in graphical window. On the left hand side pane, click on Search Results. then type cab on the bottom text box on the right pane.
   
   
   • Double Click on the desired application name. ( In this case, it is vnd.ms-cab-compressed ).
   
   
   • On the new Edit type application/vnd.ms-cab-compressed window, Click on Filenames tab. Then click on Add button, type *.themepack on the new Add filename rule dialog window. Click close, and then Exit the program.
   
   

Now you should open any *.themepack extensions file with file-roller just like any other compressed tar, zip etc files.

If you want this settings persists globally (system wide), you should open the program in the step 2 as gksu assogiate, all other procedure is same.

Credit goes to MistreLion, answerer of this post.

Update

I recently upgraded my Ubuntu to 12.04 and in this version, you need only to install p7zip-full package.

Open a terminal, type the following command , provide password and hit enter. That's the all required (of course, after connecting to the internet).

sudo apt-get install p7zip-full