I would say that you can just rename the files to your liking.

for example in the MKIV use the following path \V_2\Voices\0506

Change the usa_f.csf to what you want. for example gb_f.csf to usa_f.csf and vice versa.

That would be easier than modifying the voice.lst file and the voice.lst file checksum

Here are the choices Male and female of both

D german
E Spanish
F French
gb British
i Italina
nl not sure
rus Rusian I assume
usa us english