A Virtual Repository defined in Artifactory aggregates packages from both local and remote repositories.
This allows you to access both locally hosted binary assets and remote proxied Git LFS repositories from a single URL defined for the virtual repository.
To create a Git LFS virtual repository, from the Administration module, select Repositories | Repositories | Virtual and set Git LFS to be its Package Type, and select the underlying local and remote Git LFS repositories to include under the Repositories section.
Make sure you also set the Default Deployment Repository so you can both download from and upload to this repository.