From "Risks from GMOs due to Horizontal Gene Transfer", by Paul Keese:
In addition to direct HGT between organisms as depicted in Figure 1, forms of indirect HGT have been observed, which involve an additional intermediary organism in gene transfer from a host organism to the final recipient organism. The most notable are virus mediated gene transfer (transduction) between bacteria (Weinbauer, 2004), retrotransfer of a plasmid to a second bacterium, acquiring host genes and returning to the original bacterium (Ronchel et al., 2000), the spread of donor genetic material between several different bacteria coexisting in complex communities or biofilms (Molin and Tolker-Nielsen, 2003; van Elsas et al., 2003; Wuertz et al., 2004); or from virus to virus via sequences integrated into a common host organism.
Does it mean that a viral gene or other DNA sequence gets integrated into the host's genome and then some other virus (that chanced to be in the same organism) manages to obtain this sequence and package it into its own DNA?
Keese's article has no reference for this statement, and perfunctory googling brought nothing to clear the matter for me.