Hi LG,
No. You cannot do that.
Word Object Model does not expose task pane to us for customizing. You can only get the CommandBar named "task pane", but cannot add any controls on it.
If you are using VSTO, you can create an action pane in document level project. But that action pane will only be available in that customized document, which may not be what you want. However, I am posting the walkthrough out, for your information:
http://msdn2.microsoft.com/en-us/library/kfzd656e(VS.80).aspx
Thanks,
Ji