Could not able to find correct documentation regarding realtime (rest based) data augmentation in AAM.
We were able to pass clickstream data from Sitecatalyst through serverside sync.
We are able to setup setCustomerID call when user logs in or fills the form(where ever reveals user identity) . Hashing email id.
Now how to call rest API call to add additional attributed from offline system to the same visitor signal.
something like id="offline<hashemail>"&attr1="val"&attr2="val2". Here this is not a new signal it should enrich the existing signal with these additional attributes.
I am aware of offline data file upload but that is batch process we are looking at realtime
As I have understood your issue, you want to do the ID sync or pass data via API calls to AAM servers like real time data, but without users being visited on the page, so that you could get the data ready for targetting users when required ?
You can check our DCS API help docs : DCS API Reference
However, there is an external article also that is similar to your requirement : https://www.pedromonjo.com/2016/05/first-visit-targeting/
but the only thing that is not recommended is to use the DCS regional URL, so use your company's subdomain instead of using the DCS region URL.