There is no need to put everything into one sitemap file. You can create 2 sitemaps (or as many as you want) - from Google prospective it is the same. You can add them to in the beginning your robots.txt:
Or submit using google web master tools. For Hubspot content you can alter repeater web part or Kentico Sitemap Web part(which is essentially the same repeater) and output using simple transformation:
<lastmod><%# FormatDateTime(Eval<DateTime>("HubSpotDate"), "yyyy-MM-dd")%></lastmod>
No need to complicated - just make 2 sitemap files.