Change symbols with alt modifer
Not exactly what you're asking for, but I use 

System Preferences - Keyboard - Input Sources


to add Greek &amp; other layouts, and tick the 'Show Input menu in menu bar' option. This results in a pretty menu with keyboard layouts marked by national flag. And, a ctrl-cmd-space shortcut to cycle between them.

Also:

System Preferences - Keyboards - Show Keyboard and Character Viewers in menu bar


for all other typographical symbols