Synonym for "engrooved"

Worn is probably the word your looking for. "The keys have been worn to his fingers from 50 years of playing", or something like that.

Engrooved has more the idea of grooves being put in something. It would be similar to engrave, but it is more the idea of precision machining (A machined groove), whereas engrave is more precision art.

Response to Edit: Broken in is another one that would fit. This has more the idea of a glove that is initially stiff and becomes pliable by using it. Using it customizes it to the user.

Assuming you’re asking about the way the keys of a keyboard fit ones’ fingers, you can say,

the keys are form-fitted to the user’s fingers

• Molded may fit if this is a soft, moldable material such as plastic.
• Formed is often used this way generically.
• Sculpted or rounded may be appropriate as well.

I've heard wear worn before, but that would be more for clothing. For something durable, perhaps yielded: Over the years, the ivory piano keys yielded to the form of his fingers.