I asked something similar earlier this year (concatenating compiler
flags given as header-args property, used for linking against different
libs in different sections of the Org file). I ended with a function
that grabs the current property value and returns the value concatenated
with new value. That function can be used as a 'property value'. That's
not a nice and bullet proof solution, but works good enough to me to
generate the solutions to the psets for the lecture.