I only really tested this for Silent's Gems content, but it seems some modded items aren't convertible into goo. I'd like to be able to convert gems into goo and back, but I may likely just be a dummy and this is no longer an issue.

It's a bit awkward, but I managed to to this with datapacks.
In data > goo > aequivaleo > locked there's a bunch of json files with goop data. You can add your custom ones and they'll load.

I don't think you can add custom goo through datapacks though.

You can use data packs to add existing goo types to modded items.
But please note that this whole mod is in really early development, Xeno is still working to get the core mechanics fully functional.

I am uncertain if it is or will be possible to add new goo types using data packs, simply because goo's all have special functionality that Xeno is planning for at least the default ones to have.

We need a guide for showing people how to get custom goo values into their packs, it's relatively straightforward from what I've been told. Ironically, however, I don't know how to do it. :D

I'd also really like to see custom value submissions crowdsourced for our defaults, so if anyone wants to contribute custom value entries for mods [eg. not vanilla] or just generally make suggestions on mods, the issue tracker and PRs are a good way to do that.

Addendum to the above, in fact the "mod" qualifier for value entries isn't necessary. I'm pretty open to value suggestions in general. The mod is in a very immature state and a lot of values are horrendously undertuned. Suggest all you want.