How to set properties in CustomSecurity user

How can I set additional properties in the CustomSecurity field of a RDMUser object?

This field contains a xml string. Do I have to edit this myself?

What values can be set using CustomSecurity?

<?xml version="1.0"?>

<CustomSecurity>

<CanExport>false</CanExport>

<CanImport>false</CanImport>

<CustomRoles>

<string>aa259670-dae5-42b0-b3cb-4c4edd283ddd</string>

<string>6384f8e4-18fc-4239-8b24-c2e0749f1bb8</string>

</CustomRoles>

<DenyAddInRoot>true</DenyAddInRoot>

<IntegratedSecurity>true</IntegratedSecurity>

</CustomSecurity>