Hello Wow,
You are absolutely right, the modulation option available in the STO-PLL configuration does not currently appear in the Workbench interface for the HSO case.
This feature is planned to be added in upcoming MCSDK releases.
For testing purposes, it is possible to modify the modulation mode through the register interface using the ST Motor Pilot GUI, as illustrated in the figures below:

If this meets your requirements, you can update the default modulation mode directly in the firmware code (mc_config.c) by modifying:
.modulationMode = FOC_MODULATIONMODE_Centered,
to either:
.modulationMode = FOC_MODULATIONMODE_MinLow,
or
.modulationMode = FOC_MODULATIONMODE_MaxHigh,
for your information the supported modulation modes are:
FOC_MODULATIONMODE_Sine
FOC_MODULATIONMODE_Centered
FOC_MODULATIONMODE_ShiftedCenter
FOC_MODULATIONMODE_MinLow
FOC_MODULATIONMODE_MaxHigh
FOC_MODULATIONMODE_UpDown
Regards,
Fouad