The problem is his voice is already too entrenched as Luke.
You obviously haven't heard him as the joker. I never knew it was Mark Hamill till i saw him talk about it in an interview.
Crappy quality but it's the best one that shows the sudden change in his voice.
Voice actors need to be a little more versatile then just using their regular voice !! In theory Mark Hamill could be in SWTOR and you wouldn't know BUT i doubt that he is though.
Here is another where he does the voice of Xmen wolverine for the wolverine videogame