This is the full list of the modUser object fields:
http://bobsguides.com/modx-object-quick-reference.html#modUser. No date fields at all, I'm afraid.
The
modUserProfile object has the 'extended' field, but it's a pain to work with since it holds a JSON string that's converted to an array when you retrieve it. In order to interact with it in the Manager, you have to 'create' the field for each user and you can't easily access it directly with a placeholder as you can with the other fields.
There are a number of other User Profile fields you could Co-opt if you're not using them (e.g., phone, mobilephone, country, fax, photo, comment, etc.) You can see the length of each of them
here (in the [fieldMeta] section). It's easy to change the captions for them in Lexicon Management so you can identify the one you want when you edit a user.
You'll need a plugin to save them during registration, as you suggest. The big decision is whether to save them as human-readable dates or Unix timestamps. It's easy enough to convert from one to the other (and there are MODX output modifiers for it). The timestamp option is usually better, since you can do easy date comparisons and searches, and you can change the displayed format of them without messing with your code.
A whole other approach would be to add a new field with ClassExtender, but if you can use one of the existing fields, it will be faster and easier. If you go this route, don't use 'createdon' as the field name as it will conflict with that resource field.