There might be a possibility to make an external conversion script (outside Archetype Designer) between certain formats by using the code open sourced by Better, see links in the thread Is anyone using Marand's web-template project? I doesn't build (@birger.haarbrandt also mentions EHRbase openEHR SDK in that thread, I have not explored if that wouls cover your use case or not.)
I also believe @thomas.beale made it possible to run the ADL Workbench (AWB) tool in command-line mode so that certain conversions/exports can be scripted for batch jobs, but I am not sure AWB supports the web template format (yet).
@Owen_Far if you get some conversion script/tool working, then please share it with the world.