I think this question has been asked like months back, wondering if there was any progress on this? Got a ton of cash sat in my wallet that I'm not spending, if only I could buy Owl Cookies with it..
Yeah just go through steam, load poxnora, go to marketplace and hit buy credits. It always uses your leftover cash whenever I buy things on the pox website, most of which I get from selling poxnora trading cards and similar items.
I tried it using the poxsite and the non steam client, on the account that I still can't load up the steam client consistently (on random occasions it will work, mostly it won't). Good to know it's possible in theory though, thanks