Open a new eagle board file. In this board file go to File -> import -> eagle drawing. Select the board/boards you want to have on your panel and place them on your board layout. This imports the schematic and the board layout of the project you imported.
Once you have all your boards imported you will see that you have unrouted lines between your boards. At this part you can do two things: Ignore it and disable the unrouted layer (layer 19) or you can edit the signal names in the schematic. The last option is alot of work to do. Also the schematic becomes harder to read because your probably (atleast this is what I do) name the signals like this(I take the 5 volt line as an example): 5V, 5V_1, 5V_2 etc. Thats why you start a new eagle project for this so you can keep your schematics apart from each other and keep them readable.
Last problem you will run into is changing the names of your ground. For that I'll refer to this question. You should read my answer and be able to change the signal names for your ground then. There is a chance that you also have to change the names of you polygons of your ground planes too.
This is as far as I know the only way to do this.