How to create more than one part from one "instrument"

Let's say I want to write a polyphonic piece of music, but it's not fixed which instrument(s) will play part (voice) 1, which part 2, etc. So I write a score with 4 instruments in C. Now I can create 4 parts. Suppose part 1 will be played by instruments in C, in Bb, and in Eb. Obviously, I want to give each player the transposed sheet music. How can I add the transposed, but further identical parts, without having to add them to the main score?