1) open the library you want to add a package to
2) find the package you want to copy in the eagle 'control panel'
3) drag and drop the package into the library editor
4) add new package to parts and connect the pins
What I have done is
Create a new library and then a symbol for a 14 pin PIC device. Then I followed the above setps 1 to 3. The 4th step is confusing. How to add new package to parts and connect pins?
Edit: I have created a library for PIC16(L)F1829. I am attaching the library. Please somebody chech it and tell if it is ok or not. If it is ok. I will continue creating library for all PIC and AVR devices.