TL;DR: Make an HTTP request directly to the registry:

TOKEN=$(curl -s \
  -H "Accept: application/json" \
  -H "Content-Type:application/json" \
  -X PUT --data '{"name": "username_here", "password": "password_here"}' \
  http://your_registry/-/user/org.couchdb.user:username_here 2>&1 | grep -Po \
  '(?<="token": ")[^"]*')

npm set registry "http://your_registry"
npm set //your_registry/:_authToken $TOKEN

Rationale

Behind the scenes npm adduser makes an HTTP request to the registry. Instead of forcing adduser to behave the way you want, you could make the request directly to the registry without going through the cli and then set the auth token with npm set.

The source code suggests that you could make a PUT request to http://your_registry/-/user/org.couchdb.user:your-username with the following payload

{
  name: username,
  password: password
}

and that would create a new user in the registry.

Many thanks to @shawnzhu for having found a more cleaner approach to solve the problem.