The problem with MS Word conversions is always the inconsistent nature of MS Word documents. Even within one document the author may be using a variety of styles, tables, lists, etc.
If you had perfectly consistent MS Word documents that very closely matched your S1000D Data Module requirements then yes, this would be a straightforward activity. However, reality is that it is not usually that easy.
Having said that there are specialist organisations, like ours, who specialise in such things. We are also able to engage offshore BPO vendors to do large-scale conversions in a cost-effective fashion.