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The workaround is after VSP onboarding and import, go to Deployment Artifacts on VNF design GUI, and click Add Artifact then choose Type as Other. From there you should be able to upload your zip file. The zip file will be stored under Artifacts/Other directory of SDC output TOSCA CSAR file.
VoLTE Service Design
Step 1. Create New VSP by upload vendor VNF CSAR.
Step 2. Create VNF by importing VSP, and submit for testing