To convert to/from hexidecimal in emacs, it's advised to use M-x calculator instead of M-x calc. The single-quote key (M-x calculator-rotate-displayer) will eventually get you to the point that you're outputting with the correct formatter (instead of scientific notation). Then:

• H to switch to hex i/o
  
• C-y to yank a hex number in
  
• D to switch to decimal
  
• M-x calculator-copy to copy the number
  
• backspace to clear the number from calculator
  

Then it's just on the killring and you can C-x o over and yank it.