maembe,
clientID is a random number generated by Google Analytics, and keep in mind it's always required and its value should always be a random UUID (version 4) (you could technically use your own, but I am not sure how practical and reliable this would be). Most importantly, you can easily access it with predefined get function (see documentation).
For your needs, this is exactly what you should do -- if someone sings ups, store ClientID in your CRM and then if there is any offline purchase, record the transactions with measurement protocol using the stored clientID. Google Analytics will then make the link (attribution) with that visitor and you will see this in your reports. Also, take advantage of newly available custom metrics and dimensions which can store pretty much anything you want (think of customer segmentation etc.). Beware of storing PII though.
Hope this helps :)
I am curious how UserID is going to work, it might change everything, but for now, I wouldn't rely on it as there is very little information available.