I did a google search for 'quill seatpost' and every single one, without exception, that I found had either a built-in adjustable clamp or a reduced diameter at the top where a basic seatpost clamp (also asdustable) could go. Do you have a picture of the seapost in question as I cannot imagine a seatpost that does not allow any angle adjustments.

Like Well Biked said, there are also lots of examples of people modifying standard seatposts to use an expander or wedge.

Edit: FBinNY beat me to it with a comment about all seatposts having saddle angle adjustability. However! Some kids bikes (and maybe some ultra cheap dept. store bikes, but I cannot recall any examples) have a saddle permanently mounted on top of a straight post with no adjustability.