Per LDM-151 v4.1 §3.2.5:

External targets (e.g. well localized transient events from other telescopes or instruments) can be incorporated within this component of the nightly pipeline (essentially treating external sources as additional DIAObjects and associating them with the DIASources)

It's not clear from that text whether external objects will literally appear in the DIAObject table regardless of whether LSST has observed them, or whether there will be some other table which is consulted during source association. Please clarify.