Copy the contents of the METAL GEAR SOLID MEGAPACK's "pack_*****" folders.
Launch Choicer Voicer
Click on "Open Game Folder in Browser"
Paste into the respective folders within the Choicer Voicer "game" folder.

Example: from the METAL GEAR SOLID MEGAPACK, copy the contents of "packs_voice". Now paste said contents into Choicer Voicer's "packs_voice" folder.