XML : MS Word XML Content Controls - custom XML output

I'm trying to devise a way of taking a large number of Word docs/pages and converting the text/content within them to a custom XML output in the simplest way. The documents in question are user manuals.

I was originally going down the custom XML route whereby the contents of the Word docs could be tagged manually with custom tags, and I would then use a custom VSTO add-in to output the tags to an XML file in accordance with a schema. However, after looking into that option it seems it not possible due to Word not supporting custom tagging (as such) anymore.

The other option which looks plausible is to use Custom XML Parts. I'm still a little unsure as to whether it's a good fit for what i'm trying to do though. As far as I can see, I could add custom parts to a document like

'Component' 'Procedure' (i.e remove or install) 'Part Number' 'Spares' 'Procedural step' 'graphics'

But the problem is, I don't know how many, let's say, 'Procedural steps' there will be for this particular component. It could be one or 20. If there were 20 then I would need 20 XML elements in the output. Similarly there might be a whole list of 'spares' in one doc, and none in another, same with graphics, so the XML mapping needs to know when to add more elements, or not add elements where not necessary - so the output XML has the correct number of elements.

Does anybody know a good solution to what i'm trying to do?

Thanks.