How about an XML Stylesheet?

https://stackoverflow.com/questions/24122921/xsl-to-convert-xml-to-json

https://stackoverflow.com/questions/43355563/convert-xml-to-json-using-xslt

You need to provide subscript values for the three loops.

This will give you the 1st member of each collection:

source.{loop2000A(1).loop2000B(1).loop2300(1).CLM:ClaimSubmittersIdentifier}

It's likely that in a real DTL you'll want to loop over each collection, because there will probably be multiple claims in the message.  Use ForEach to do that:

<foreach property='source.{loop2000A()}' key='k2000A' >
    <foreach property='source.{loop2000A(k2000A).loop2000B()}' key='k2000B' >
        <foreach property='source.{loop2000A(k2000A).loop2000B(k2000B).loop2300()}' key='k2300' >
            <assign value='source.{loop2000A(k2000A).loop2000B(k2000B).loop2300(k2300).CLM:ClaimSubmittersIdentifier}' property='target.ClaimInvoiceNo' action='set' />

         </foreach>
    </foreach>
</foreach>

Note that the way I have that now you'll end up with the last ClaimInvoiceNo in your target.  You'll need to adjust to make sure you process each of them.