How use 'Hierarchical Blocks' in Eagle?.. nesting schematics within schematics?
I didn't know it could do this, but I've just tried it out, and it works. Here's how I did it.


Open a new schematic, or whichever schematic you want to add an existing design to.
File->Import
Select the schematic of the design you want to add to the open schematic.
Make sure the nets are named the way you want (identically named nets will merge)


That's it, I get a new sheet for the added schematic and the completed layout added next to the existing layout.

If this doesn't work for you, it's likely because you're using the Eagle Freeware, not Eagle Standard or Professional. The hobbyist/non-commercial version is not too much if that is the problem and it counts as Eagle Standard. It's what I have.