Probably unrelated, but you most likely do want MSI.IN835.EOBList to have storage.

The reason is that if your BPL suspends for any reason (such as a Call) between the time you set and use that data, you'll lose whatever values you were trying to save.  That's because the job that's executing your BPL will %Save the Context object and go work on another task while it's waiting.  When the Call returns it will reload the Context object and resume work.  If you extend %RegisteredObject your context property won't survive the save/reload.

It might be tempting to ignore that if you're not currently doing any intervening Calls, but things tend to change over time, so doing it now could prevent a hard-to-find bug later.

%SerialObject is probably better that %Persistent for this because that way you won't have to implement your own purge of old objects.

Or, if you only need to store a list of integers, you could just declare your context property as that and skip the custom wrapper class.

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.