depending on your OS / windowing system, you actually have easy ways to quickly get greek letters out of your (non-greek) keyboard without having to copy-paste them: my favorite way is to assign a key or key combination of your choice to act as so-called dead_greek modifier, and then just press that modifier before a latin letter key to get the corresponding greek letter. For example, under linux/xorg, if you wanted AltGr+g to be your dead_greek, you can use xmodmap to set it so:

  xmodmap -e "$(xmodmap -pke | grep  "keycode\s\+42" | sed -E 's/=((\s\S){4})\s\S+/=\1 dead_greek/')"

you'll find more on that here: https://askubuntu.com/questions/787113/compose-dead-greek-wi...

alternative 1: assign a key as a direct modifier (not a dead key) and add key combination defs for that to generate the characters you want … alternative 2: switchable keyboard layouts

I set my compose configuration. I just write <composekey>delta = δ, or <composekey>DELTA = Δ (and so on for the rest of the alphabet).