A real piano is a huge asset to any recording studio. Barefoot, where I work, has a beautiful Knabe grand piano that is used often. If you are stuck using digital keys, there is still hope.
1. Try pumping the digital keys back out into a room and recording the sound of the room. Blend this in to the original digital keyboard track. This works very well if it is the same room that other instruments on the track were recorded in. It helps to “glue” all of the elements together with a cohesive ambience.
2. If you are mixing in a studio with a grand piano, and you want to keep your awesome piano performances, you can re-amp the digital piano signal back into the piano enclosure. This will excite and vibrate the wood and strings of the piano, which is often the element lost in a digital representation of a piano.
3. If you are using fender rhodes, wurli, or synth tracks, it can be a fun effect to run those through the piano enclosure also. Think of it as “pianoverb”
If you want further information on how to get your digital piano tracks to hold up to the real thing, send me an email firstname.lastname@example.org