Character (run-> charmap) map lists all the unicode codes for the symbols. Eg capital pi is 03A0.
So get that number and convert it from hex to base 10 in windows calc (run -> calc) then alt+blah the resulting number
Or you could just drag and drop from charmap. ΠΔμαβδ etc etc.
Another way is as TM was saying with the &#xxx; you can also use the names of the chars to type it in. ie xxx = gamma gives &#gamma; <--Edit: Hmmm, strange, that used to work in vbulliten.