Connecting Custom Metric Part in Eagle
Basically for compatibility with everything else, you should always lay the pins on your schematic symbol on a 0.1" grid. Either set the grid in the symbol editor to be 2.54mm or set it to be 0.1".

If you need to make a metric footprint you can do so, use whatever grid you want to in the package editor. 

The packages don't appear in the schematic so you don't need to worry about them being on-grid. The symbols don't appear in the layout so you don't need to worry about them not matching the dimensions of the part.