App flexibility - summing feeds, for example

I’ve been using the June15 system (8.5 low write). The apps on that system seem more flexible than the equivalent on to which I’m sending data. On the 8.5 version, for example I can specify multiple feed to sum for the feeds. On the .org system, I’m limited to one feed. I think. As I have three phase mains and also a three phase solar pv system, I’d like to be able to sum the feeds. I seem to think I read there is/was a way to create a virtual feed which basically performs some processing on ‘real’ feeds, which I could use for summing, but I can’t now find it. Can anyone point me in the right direction? TIA.