Here is how you can play custom campaigns:

[Malistroth]

I'm writing this for the people who may not know.

1. Open the campaign via Campaign editor in WE.
2. Extract the campaign maps into a folder on your desktop.
3. Put the folder into C:\Users\%USERNAME%\Documents\WarCraft III\Maps (that's for Windows 7)
4. Start the WarcraftIII.exe
5. Go to the "Single Player" -> "Custom game"
6. Find your folder and start any mission from the campaign.

The progress of your heroes will not be saved through the campaign, but that's what we have atm.

 

[Footman16]

Wouldn't you then need to re-import any campaign imports into all the maps and edit all the objects in the object editor to use the War3mapimported/ import path?

 

[Malistroth]

Wouldn't you then need to re-import any campaign imports into all the maps

If it has a lot of import - yes, you would.
But what I wanted to say: campaign authors can make new content for reforged or port it there. So we can play the campaigns.
Some campaigns have imported files directly into the map files, not into the campaign files.

 

[Razorclaw_X]

This method does nothing for anything implemented at the Campaign level-- especially the Object Editor, which you almost certainly can't just import them into every map and hope everything will work correctly.

 

[Warseeker]

I don't know if Reforged support custom MPQ but if it does then one can create a custom MPQ containing all the campaign assets and put it inside the Warcraft 3 folder. That way you can see all the assets without having to import anything. However, this will require the player to download the map and the data separately...

 

[deepstrasz]

I don't know if Reforged support custom MPQ but if it does then one can create a custom MPQ containing all the campaign assets and put it inside the Warcraft 3 folder. That way you can see all the assets without having to import anything. However, this will require the player to download the map and the data separately...

I don't know about .mpq but that's how StarCraft II works with things called dependencies, a pack of stuff that some maps require to be played properly with all the custom assets used.