What you do it totally export the wil you want to add files to, to a blank folder. Take note of the last image number.
Totally export the images you want to ADD to a seperate blank folder and use a minus offset to make the image number line up.
E.g. the last image number of the first will is 50, the image number of a range you want to add starts at 100 so the offset would be -49. This means the start number of the second exported wil images would be 51 onwards.
Copy the images from the second wil into the folder of hte first. This should NOT prompt overwrite messages if the offsets have been done right.
Add the entries from the second wil 00000.txt to the first wils 00000.txt
Choose build wil from menu and select the directory with the merged wils. Name it as appropriate.
It is complicated but this is the best way to do things.