Adding fields to item in source_type.xml

[crantron]

I'm trying to add a field to the FTP item in source_types.xml

I tried something like this in my Custom_Module:

Custom_Module/etc/source_types.xml

<?xml version="1.0"?>
<!--
/**
 * @copyright: Copyright © 2017 Firebear Studio. All rights reserved.
 * @author   : Firebear Studio <fbeardev@gmail.com>
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../vendor/firebear/importexport/etc/source_types.xsd">
    <type name="ftp">
        <field name="completed_import_job_image_location" id="completed_import_job_image_location" label="Completed Import Job Image Location" type="text" required="false" />
    </type>
</config>

But obviously that wont work after looking at the related source_types.xsd. However if I add it to vendor/firebear/importexport/etc/source_type.xml, flush cache it works fine and I know you're not supposed to do this but this was just proof of concept. Should I be using a plugin, or some other method?