That's true, saying 'you have to do this' ir much easier than doing it
What makes doing this tricky without a tool is that general format for WLD files is like this:
[header]
[string table]
[fragment 1]
[fragment 2]
...
[fragment n]
In theory you could just add the new fragments at the end of the file using a hex editor and edit the fragment count in the header (sounds tedious an error prone though). The problem is that you need to add the texture file names to the string table which forces you to shift all of the fragment data that follows. I'm not sure if hex editors let you do this.
This is probably doable by writing a script that loads the WLD file, adds the fragments, update the string table and write everything to a new file. I'm afraid that doing this by hand is not feasible.