With Far Harbor and Nuka World installed, are any items added to the game's loot table from the beginning, or are they only available after traveling to the DLC locations?
Like @SGR said for the majority of the loot that you find in the base game area will not be from the DLC. However, I have seen legendary drops which are from one DLC that will either show up in a different DLC and I believe also in the base game area. For instance, I once found a lever-action rifle in the Nuka DLC before starting Far Harbor.
Hope this helps!
If you only have Far-Harbor and Nukaworld installed, I don't think you'll find any loot from the DLCs in the base Area. However, It happened to me to find DLC loot dropped by ennemies found in both DLC and Base areas, while killing them in the Base area, for exemple Raiders, found both in Nuka work and the base Area, but it might just be NukaWorld that added those ennemies.
Started a new game recently and I dropped a Far Harbor weapon in the base game (around level 20). This seems to be very rare though and I haven't found any ammo for it yet.
The only DLC quest I have for now is the Automatron one, so Far Harbor is not accessible yet. All DLC are enabled.