You can install an SDK directly from a local tarball using the --tarball flag. You need to specify the SDK --type when you use this flag. For example:
nrfutil sdk-manager sdk install --tarball C:\downloads\nrf-sdk-v3.2.0.tar.gz --type nrf
The SDK version is read from the VERSION file within the bundle, so the SDK type must be specified to locate it.
The installation process unpacks the tarball first to find the version and then determines whether the installation can proceed.