You can combine designs onto a single board.
In the board editor, you can create a new board without an attached schematic -- and then copy all the separate layouts into this combined board. It is not necessary to copy all the schematics into a single schematic file.
One problem that occurs is that EAGLE will change reference designators on the silkscreen as necessary to avoid duplicates. The solution is to follow the same steps you would use to panalize a single board design. Run the panelize ULP (File > Run ULP... or Tools > Panelize) to create "shadow" silkscreen layers that contain the original reference designators before copying. These new layers (125 and 126) will allow duplicate reference designators.
Section Combining Small Circuit Boards on a Common Panel in the EAGLE user manual provides the detailed steps:
Load the board file.
Run panelize.ulp to copy name texts.
DISPLAY all layers.
Use GROUP to select all objects to be copied. To select the whole layout you could also use GROUP ALL.
Click the COPY icon in order to put the group into the clipboard.
Edit a new board file with File/New.
Use PASTE and place the layout as often as wanted. If necessary, it is possible to specify an orientation for the group before fixing it.
Please make sure that the new board has the same set of Design Rules as the original board file has. It is possible to export Design Rules into a file (*.dru) and then import it into another board file (Edit/Design rules menu, File tab).
Save the new board file.
Tell your board house that they have to use layers 125/126 instead of 25/26.
If you are planning to have the board scored or routed (with breakaway tabs) to make the boards easier to separate, those details -- and the overall board outline -- can go on the Dimension layer of the combined board file.