I made my own frame based on another one from the library. It's actually very easy.
Once you copied the frame/library and renamed it to something new, you open the frame in the device editor.
Than you open the symbol (the actual frame) in the symbol editor. Now make the changes you want.
There are some predefined words that will be automatically replaced by Eagle with actual data like:
>DRAWING_NAME (filename)
>LAST_DATE_TIME (date/time when the file has been modified)
>SHEET (sheet number)
For example, if there's written (in the symbol) "Sheet: >SHEET" (without the quotes),
then, after adding the frame to the schematic, there will be written "Sheet: n" where n is the sheet number.
After adding the frame to your schematic, you fill in the empty boxes with text strings.