In short, no.
The best you'd be able to do, and then only if you wrote a new ...uh... custom customfield plugin that set itself based on the issue's key. However then you'd have further problems deciding how to handle what to do when the issue is moved to another project (and gets a new key).
I'll ask as I'm sure someone else will - what problem are you trying to solve? enable easy location of the parent issue? easier than just clicking the parent issue link? provide some more details, perhaps someone will have a better idea
EDIT: the issue's key is created from the issues project key followed by a numeric, and JIRA enforces this to look like 'ALPHA-ALPHA+'-'NUMERIC', and you can't change that without taking on so many more changes that the problem you are actually trying to solve disappears in the lost requirements of a forgotten dream.
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