I'm assuming that you've used a resource editing tool before, if you haven't this isn't a lesson, sorry.
The Error tables are held in resources of type 'eTAB'. The resource number doesn't matter, but the resource name is used in the menus. Make sure that you don't have multiple resources with the same name, the menu selections may become confused. The template that is supplied in the application resource fork for the error tables is compatible with both ResEdit and Resorcerer.
Possible problems you may encounter...
ResEdit may not be able to open exceptionally large tables, ie, greater than 16k.
If you add masses of error tables you may need to increase the application memory allowance.
Using a resource compiler
There's one in every development environment, MPW has Rez, both Symantec and CodeWarrior have built in compilers. There's a version called SARez kicking about somewhere. I think it used to be supplied with Think C.
Here's the resource definition, you should be able to copy it from here and stick it in a file called 'eTAB.r' or something similar.