I think what you’re saying is that SPICE isn’t yet registered as a Julia package (meaning it doesn’t have an entry on the general registry such that Pkg knows what you mean if you just type add SPICE).
I’m not an expert in unregistered packages, and when I tried to do something like this I gave up and squashed everything into a single unregistered package that I dev’d and called via using in a script. But the following might help:
https://discourse.julialang.org/t/pkg-workflow-for-unregistered-packages/17385