Do you mean like taking a letter cutout and sticking it in clear resin? Then sticking the mold together to lets it dry?
If that's what you mean then I would just say no because you would have a high chance of the letter migrating when you put the mold together and it drying sideways/crooked/upside down/etc...
You would really need a dual cast process for it to look really good. I'll make a doodle.
Step one:
you would need to make the top mold be the smooth exterior of the button and this same mold could be used for both steps.
The bottom mold would need to form the transparent outer shell (in the color of your choice) with a letter indention into the shell. Like so...

Step two:
Like I said, we would use the same top mold, but make sure you leave your previous shell in the mold because as shown in this step you will be pouring a solid color (white would be the best choice to make the letter most visible) inside the shell.
The bottom cast would look like the normal inside of a button. Like so...

This is how you would get your cool looking xbox style buttons.
I know it looks a bit crude because it was like a 10 second drawing and I'm really freaking tired., but I hope it gets the point across.

YAY!