setopt magic_equal_subst		# for `export foo=~/bar`
function export() { EXPORT=${1%%\=*} ; typeset -Ux $* }
function +() { eval builtin export ${EXPORT}=\$\{$EXPORT\}:\$1 }

# now you can do
#
# export PATH=/home/$USER/bin/$OSTYPE	# arch dependent binaries
# 	  +	/home/$USER/bin		# arch independent scripts
# 	  +	/usr/local/bin		# local apps
# 	  +	/opt/gnu/bin		# gnu tools
# 	  +	...
#
# or even stuff like:
# 
# export PATH=~/bin/$OSTYPE		# arch dependent binaries
# 	  +	~/bin			# arch independent scripts
# 				      case $OSTYPE in
# 				      sun*)
# 	  +	$OPENWINHOME/bin	# openwindows applications
# 	  +	/usr/ccs/bin		# compiler tools
# 	  +	/opt/bin		# optional packages
# 				      ;;
# 				      *)
# 	  +	/usr/bin/X11		# X11 applications
# 				      ;;
# 				      esac
# 	  +	/usr/local/bin		# local apps
# 	  +	/opt/gnu/bin		# gnu tools
# 	  +	...

unfunction export +