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);
};

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
}