This Moodle question import format can import questions exported from a STACK 2.0 system into Moodle with STACK 3.0 installed (https://github.com/maths/moodle-qtype_stack/). It was created by Matti Pauna and Chris Sangwin.
To install, either download the zip file,
unzip it, and place it in the directory moodle\question\behaviour\stack.
(You will need to rename the directory moodle-qformat_stack -> stack.)
Alternatively, get the code using git by running the following command in the
top level folder of your Moodle install:
git clone git://github.com/maths/moodle-qformat_stack.git question/format/stack
For full install instructions, see the STACK install instructions.