No, only the same sample rate for anything is supported. You will need a sample rate converter for your digital keyboard.

The RME will work fine with inputs that run at a common frequency (and assuming master/slave clocking is configured correctly).  If your keyboard has an ADAT output, the RME will gladly accept it as long as you have everything configured correctly (and any other digital gear you have in the system must be configured correctly as well).

The issue comes when you want to run your RME at 48K, but have a Keyboard (or other digital box) feeding at 44.1K (or RME @ 44.1K and Keyboard at 48K, etc).  Like MC already stated, you will need a standlone sample-rate converter in that case to get everything sychronized correctly...

The keyboard might have adjustable samplerates as well - and might even be able to slave to an external sync source (Wordclock, ADAT Input, etc).  Alternatively, you can bring the Keyboard in over Analog if you must (avoiding all samplerate mismatch issues).

What is the exact scenario you face?

Looks like the ZSYS Z-1SRC is an ideal (and suprisingly afordable!) candidate.  You'd feed it a BNC Wordclock connection from the RME, and the Keyboard will basically run on internal clock (free run), and the ZSYS Z-1SRC will convert it "on the fly" to match the Wordclock provided by the RME (dropping and/or padding samples as needed with some fancy DSP SRC involved). 

In this case, the RME and the Keyboard would both be operating as "Master/Internal Clock", but the ZSYS Z-1SRC is acting as the middle-man/translator to keep everything clean and sync'd...

It also looks like the ZSYS Z-1SRC *might* also act as a format converter, so you might be able to hook two keyboards up to two separate ZSYS Z-1SRC's, and you might be able to bring one keyboard into your RME over COAX SPDIF, and one in over Toslink SPDIF (assuming you have a free ADAT/Toslink port on your RME).

I guess there is no way to have the keyboards sync to the RME @ 48K?

Hope it pans out!  You could also go analog - some people (lots of Mastering Engineers for example) prefer the sound of a good DA>AD loop over digital SRC for these types of scenarios!  Just because the SRC happens in the Digital realm does NOT make it 100% "lossless".  Data is being shifted around and dropped, etc.  Really depends on the SRC Algorithm and the DA/AD converters in question...