No not really since the ResourceBundleBootstrapComponent only accept property files, or as in the java api described as "baseNames" (the base name of the resource bundle, a fully qualified class name).
Are you are this is something you want? You might accidentally add a new property that makes sense in the repo but may that overrides a msg key that is used in Share?
If you are sure you want to do this I suggest making it a packaging exercise that copies the .properties files both to your repository AND Share distributions.
To make your file get picked up in Share simply add your custom .properties files to the list of resource bundles in alfresco/web-extension/acme-slingshot-context.xml:
<bean id="acme.custom.resources" class="org.springframework.extensions.surf.util.ResourceBundleBootstrapComponent">
<property name="resourceBundles">
<list>
<value>acme.messages.common</value>
</list>
</property>
</bean>