That is rather difficult, but it can be done. The de2 has a adc, but I don't remember how many. Another way is giving all the data parallel, this involves using external adcs. then connect the x&y axis outputs to the dac, the z axis is a button I believe, and thus can go directly to a GPIO pin, with an external pull down resistor. The ground to ground and the supply to a supply pin, but be carefull about the voltage here, it should not exeed the max ratings of the gpio pin AND adcs.
Is this the DE2 card you have: http://www.terasic.com.tw/cgi-bin/page/archive.pl?language=english&categoryno=183&no=30&partno=1If so, it has no analog inputs or ADC device, it is digital input only. So if you really want to use that joystick you will have to implement an external ADC (maybe using a small arduino board) and then having that board process the analog data to digital communications (like SPI or some such) that can connect to your DE2 card.