Diatonic transposition within a key

[Tim McNamara]

I’ve been hunting around (without success) for a method to transpose a melody 
up or down an interval diatonically.  It’s easy to transpose into a new key to 
deal with transposing instruments, but I want to shift a melody up a third 
within the original key to create a harmony line.  Does Lilypond have that 
capability to save me having to do it manually?

Thanks!