The typical way to set alt text (aka alt tags) to assets is manually. See:
https://www.youtube.com/watch?v=5AEFSXn--Ek
However - one way to automatically add these alt text values to assets - you will have to write a custom service. You will have to use QUeryBuilder or JCR SQL2 to search for these nodes. The property that stores these values is shown in this illustration:
[img]damTags.png[/img]
You set them in the service by using JCR API. TO learn how to set node property values using the JCR API -- see:
https://helpx.adobe.com/experience-manager/using/programmatically-accessing-cq-content-using.html
use the node object's setProperty method.