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:
Sitemap: http://www.mysite.com/sitemapindex.xml
Sitemap: http://www.mysite.com/sitemapindexhubspot.xml
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:
<url>
<loc><%# Eval("LocalHubSpotKenticoURL")%></loc>
<lastmod><%# FormatDateTime(Eval<DateTime>("HubSpotDate"), "yyyy-MM-dd")%></lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
No need to complicated - just make 2 sitemap files.