You can add whatever you want to a description line because there is no set format.
As an example, I have a script here called add_source_organism_info_to_FASTA.py that adds the organism information in brackets to the end of a description/header line if it isn't there. It uses the GI number to look up the Genbank entry and get the information from there and so it has to have that or it fails. Because it needs some information already there and there isn't a set format in FAST header/description lines, it isn't universal. And, right now it is using for accession a tag that is being phased out and so needs updating to rectify that. However, as an example of adding things the the description line, it is valid.