Asp.net - Insert A New Entry In XML?

I have an XML below like this and I want to add in another entry to it:

<?xml version="1.0" encoding="utf-8"?>
<CampaignRewardsVoucher>
<VoucherCode>Vouch001</VoucherCode>
<Quantity>3</Quantity>
</CampaignRewardsVoucher>

I have the above xml but I want to add Vouch002 after Vouch001:

<VoucherCode>Vouch002</VoucherCode>
<Quantity>3</Quantity>
</CampaignRewardsVoucher>

I have the code below which checks if an input is a duplicate and update accordingly, if not I want to create a new Vouch002 entry

'Create XmlWriterSettings
Dim settings As XmlWriterSettings = New XmlWriterSettings()
settings.Indent = True

[Code].....