Starting with 15.4 there is the ability to XOG incremental matrix data. There is a flag on the XOG file for Incremental. With this, you can modify existing rows and/or insert new rows.
<?xml version="1.0" encoding="UTF-8"?>
<NikuDataBus xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../xsd/nikuxog_matrix.xsd">
<Header action="write" externalSource="NIKU" objectType="matrix" version="12.1.1.1047"/>
<matrices>
<matrix defaultCurrencyCode="USD" name="RMEXYZLaborMatrix" incremental="true" notes="RMEXYZLaborMatrix" type="Cost/Rate">
<columns>
<column name="chargeCode"/>
</columns>
<matrixRows>
<matrixRow actualCost="1" chargeCode="*" currencyCode="USD" fromDate="1990-01-01" rate="5" standardCost="3" toDate="2017-12-31"/>
<matrixRow actualCost="1" chargeCode="rmexyz00002" currencyCode="USD" fromDate="1990-01-01" rate="18" standardCost="3" toDate="2017-12-31"/>
</matrixRows>
</matrix>
</matrices>
</NikuDataBus>