Use sempre Add, pois a opção Set-MpPreference irá substituir o existente
Para adicionar:
Add-MpPreference -AttackSurfaceReductionRules_Ids 6a863a9-875e-4185-98a7-b882c64b5ce5 -AttackSurfaceReductionRules_Actions EnabledPara consultar:
Get-MPPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Ids