The AppleScript will add the first trigger of a specified macro to all the macros in the specified macro group (you'll get a bogus extra one in the original macro if it is in the group as well, just delete that manually).
tell application "Keyboard Maestro"
set s to macro "Macro 1" of macro group "Test Macro Group"
set t to trigger 1 of s
repeat with m in macros of macro group "Test Macro Group"
copy t to end of triggers of m
end repeat
end tell