Local administrators should be able to add this by editing the Mediawiki namespace. I think the relevant messages are [[:fr:s:MediaWiki:Proofreadpage index attributes]] (for the input form) and [[:fr:s:MediaWiki:Proofreadpage index template]] (for displaying the information).

If this is something that all wikis would want it is possible to change the default messages for the ProofreadPage extension. But I think many Wikisources still needs to add this locally to see any change since they have already customised the messages.

beau wrote:

It can be changed on MediaWiki:Proofreadpage_index_attributes.