How to tell renderwith where to find template when in CMS-environment?

<%- if @topic_view.topic.tags.present? %>
<%= t 'js.tagging.tags' %>: <%- @topic_view.topic.tags.each do |t| %> <%= t %> <%- end %>
<% end %>

SS4

I wrote a backend module that uses template files to generte PDFs. Sadly the renderWith function can only find my pdfTemplate.ss if it is in vendor/silverstripe/cms/templates/mynamespace/mytemplate

Which means it would be overwritten on the next update…

So how to configure renderwith to look for template in root/themes/mytheme/templates ?

thank in advance
fabian