Can you be clearer what you mean by custom voices.
If you're talking about expansion packs loaded by YEM, then as long as there is enough space in the S770 for the waveform then the YEM data is comapatable between the models.
If you're talking about preset voices that you've customised through voice set, and saved to USB, then it depends on whether the original voice was also present on the S770. If it was then all is fine, if it was not then the S770 will pick an alternative wafeform.