Yes, you can also use your own soil and seeds. There is also the Experimental Pack that contains only the soil and fertiliser and you can then add your own seeds. (but it is a bit expensive for soil only).

We mix coconut soil with light soil and add Hauert fertiliser pellets - and of course the desired seeds, works wonderfully and costs almost nothing in comparison. But yes, it's crazy how much a bit of seed, soil and fertiliser is sold for - maybe it's the valuable plastic packaging ;)

I think you can use your own seeds without any problems. There are also the "Grow Anything Plant PODs", so you have the pods and the soil from "Click & Grow" with nutrients. But there's nothing to stop you using your own soil. Just make sure that you don't overfill the PODs and use nutrient-rich soil.