It has already been stated that while we do allow DLC for consoles (also for steam/gog) that it is for major DLCs and not for skin packs.
If we are talking about DLCs for PC games... well, what is older than Oblivion Horse Armor? Even that "first DLC to fit the description" isn't allowed per our rules for listing DLC items.
If we are talking about add-ons (that are not actually DLC) but were available for download for PC/Mac/Linux games, they can be added into PC Digital Download... but it would need to be significant. For example, I wouldn't add all of the individual maps or models (PK3) I have for Quake 3, but perhaps Rocket Arena or Superheroes would count as they are larger mods or TC/Total Conversions. It is not something I would do myself, as I do not consider those to be "collected" in the same sense. In a similar sense, I would not add emulators (such as MAME or NESticle) to PC Digital Download either.
And if anything is to be added into PC Digital Download, you MUST put into the description where the thing was available from. Otherwise, the items will get merged with Steam/GOG or removed if origination is not documented or easily determined.
If you can give some specific examples of things you would want to add, post them and we will see if they can fit into our DB in some way.