Generally you can't pin a portable app to the taskbar since the AppID of the launcher is different from the actual app executable, therefore the windows taskbar can not group them into one place.
I found a solution to this problem here:
Here is an easier version (I think):
1) Download mkshortcut from here: http://www.mediafire.com/?gzkgthmjhyv
or from the guide above
2) Extract the zip file; you will only need the mkshortcut.exe file inside the release folder.
3) copy mkshortcut.exe to your portable app main directory.
4) (Shift + Right Click) inside the your portable app main directory
Click "Open command window here".(Don't right click on any icons)
5) Type something like the following into the command line window:
mkshortcut.exe -output [Shortcut Name].lnk -target "%cd%\[Program Launcher].exe" -appid "[AppID]"
Shortcut Name = The name of the shortcut you want to create.
Program Launcher = The portable launcher name (usually ends with the word Portable.exe)
AppID = you can type the application main executable path instead of the ID.(application executable is usually inside the App directory)
Example: for Firefox Portable
mkshortcut.exe -output Firefox.lnk -target "%cd%\FirefoxPortable.exe" -appid "U:\Programs\Internet\Browsers\Firefox\App\Firefox\firefox.exe"
6) Click Enter and a shortcut will be created for you inside the portable app main directory.
7) Drag & Drop the created shortcut to the taskbar to pin it.
and that is it, now you are done.
Extra Tip: When installing a portable app don't put the version number in its path so you wouldn't have to do this every time you update the app.
Note: I know that pinning portable apps is kind of weird, but if you are like me, someone who doesn't like installing all programs every time you re-install windows, then this should be useful.