Hi Catonano,
I would be be happy to help you with this, but tbh, I am not comfortable discussing this in-depth on guix-devel, as this seems antithetical to Guix' goals.
All I will say here is that you need to adapt the npm importer to use the sources from the npm registry instead of resolving to any 'upstream' urls.
I believe Jan's importer was already able to do this last time I checked, so you might really only need to checkout their branch and rebase on current master.
Regardless, the biggest issue that remains is still that npm-land is mired in cyclical dependencies and a fun-but-not-actually unique dependency resolving scheme.
I am currently working on a guile version of what Sander did for Nix for importing entire npm dependency trees, but this will likely lead to lots of programmatically
defined packages instead of the guix approach of mostly-manually defining each package. It might therefore be a good candidate for a guix channel, if that is still
being worked on.