Happens exactly like that in this forum, too.

Well, I can answer that for myself: it worked!

127.0.0.1:6379> HGETALL user:28
 1) "username"
 2) "test21"
 ...
43) "customField1"
44) "123"
45) "customField2"
46) "xxx"
47) "uid"
48) "28"

I could then fetch this field e.g. using:

user.getUserFields(data.uid, ['customField1'], function(err, userData) {
	// do something with userData.customField1
}

Well, I have read some plugin code already, but I have yet to write my own one.

You mean like so?

In plugin.json:

	...
	"hooks": [
		{ "hook": "filter:user.create", "method": "createUser" },
		{ "hook": "filter:user.whitelistFields", "method": "whitelistFields" }
	]
	...

In my plugin library.js:

plugin.createUser = function(data, callback) {
	data.user.customField = data.data.customField;

	callback(null, data);
};

plugin.whitelistFields = function(data, callback) {
	data.whitelist.push('customField');

	callback(null, data);
};

I am integrating NodeBB into a site which also takes care of registration and login. nodebb-plugin-write-api and nodebb-plugin-session-sharing have worked well for that purpose so far.

Now I need to store some additional fields with a user to interact with the main site. One example is fetching the user's avatar from there.

The write-api documentation states for the v2 Create new user endpoint, "Any other data passed in will be saved into the user hash". Unfortunately, this applies only to NodeBB-known fields: it calls Users.create from the NodeBB source which only looks at NodeBB-known fields. Own fields, like "mySiteUserId", are not considered.

What is the best way to enhance the user with own fields then when using write-api?

I already had a look at nodebb-plugin-ns-custom-fields but might be a bit too much, as I don't need these fields to be user-editable. I could imagine that there's a much easier way?