Made some progress

app/services/setup_local_account_service.rb

@@ -0,0 +1,14 @@
+class SetupLocalAccountService
+  def call(user, username)
+    user.build_account
+
+    user.account.username = username
+    user.account.domain   = nil
+
+    keypair = OpenSSL::PKey::RSA.new(2048)
+    user.account.private_key = keypair.to_pem
+    user.account.public_key  = keypair.public_key.to_pem
+
+    user.save!
+  end
+end