Yes, it is possible to add sources to an existing iKnow domain, regardless of the type of sources.
Which method are you using to create iKnow domains : iKnow architect, subclassing %iKnow.DomainDefinitions or Loader & Lister ?
For the first two methods, you have to make sure you do not drop the domain before running the Build again.
iKnow will not process existing ID's in a domain when you launch the build a second time without the Drop option. It will issue warnings (Source with external ID already exists) but continue to index the rest.
For the third method (with loader and lister), you can simply take the existing examples for creating a domain, but ommit the .DropData() part.
I always use a SQL lister with a Where clause to filter only the 'new' records to add.