How is it possible to customize toolbar of finder & add more buttons other than defaults ?
Which kind of buttons are you talking about?
If you want to customize your toolbar by adding or removing existing buttons:
[This also works for most apps, btw.]
If you want to add new buttons that aren't shipped by Apple:
In case it wasn't already clear, you can also drag folders or files to the toolbar:
Hmm, can't post image, so you will have to use your imagination :-(
Or wait, nm, they just can't be inline:
[EDIT] okay, now it can be inline
I started using OS X back in 10.1, well before the sidebar came out, so that's the way I used to get around. To add these, drag the item to the toolbar (when you're not running the toolbar customization sheet. Adding a separator bar from the customization sheet will make dragging things to the toolbar easier (aim for the divider, then the Finder will know you want to add the item to the toolbar, rather than, say, copy the item to your Applications folder).
A couple of other tips if you weren't already aware of these. Hold down Command-Option and click on the toggle-toolbar button (the oblong button in the upper right) to immediately invoke the customization sheet.
You can also hold down the Command key to allow manipulation of the toolbar items without having to use the customization sheet. For example, if I no longer want my Apps folder in the toolbar, hold down Command and drag it out of the toolbar and release, and it will poof like when you remove an app from the Dock. Or, you can hold down the Command key and drag the toolbar items to the left or right to rearrange them.
Also, hold down the Command key and press the toggle-toolbar button to cycle through the different toolbar viewing modes (for example, text or icons, and large or small size, etc.).