This dialog is available from the menu at Help / Utilities / Combine Split ZIP....
Use this command to re-combine ZIP files that were previously split. Split ZIP files will have a number added to the end of their filenames. For example: Backup Job 1.zip.001, Backup Job 1.zip.002, Backup Job 1.zip.003, etc... Use the Browse... button to select the first file (the one ending in .001). All split parts need to be located in the same folder with the *.001 file. Press Combine to perform the re-combination. The original split parts will not be modified. The re-combined ZIP file will be placed in the same folder as the split parts, and be named without any number extensions, for example: Backup Job 1.zip.
In the event you have split ZIP files, but don't have access to Safe Data Backup, you can re-combine the split parts with a variety of other tools. You are not required to use Safe Data Backup for the re-combination. Any tool that will simply append each part to the prior one will work. For example, you can do this from a command prompt with: copy /b "Backup Job 1.zip.001" + "Backup Job 1.zip.002" "Backup Job 1.zip"
Split Compressed ZIP