Auto-include new metadata components in 1st-gen managed package

Question

Although managed packages automatically include new metadata components which are explicitly referenced by one of the components in the existing package, I frequently find myself manually adding new test classes to the package after deploying them to the packaging org.

My question is, is it possible to automatically include new metadata components in a first-generation package when deploying the new metadata into the publisher org? Perhaps through some configuration in the meta.xml file?

I should note that I've tried adding a <fullName> tag to the test class' meta.xml file, but observed that the class was still not added to the package despite its namespace being correctly populated.

UPDATE I've unsuccessfully tried deploying via package.xml with a <fullName> parameter, here's the package.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <fullName>RDNASequence</fullName>
    <types>
        <members>DummyApexTest</members>
        <name>ApexClass</name>
    </types>
    <version>48.0</version>
</Package>

Here's the result in the packaging org:

Here's a screenshot of the name for sanity check:

Answer 1

Yes, you can: the key is the package.xml file, though, not your individual -meta-xml files.

If your package.xml include a fullName component with the name (not namespace) of the target package, components you deploy will be added to the package, e.g.,

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <fullName>YourPackageName</fullName>