KeywordQuery with Added Filtering on Custom Fields

Greetings,

We have a SharePoint document library that we can query by using the KeywordQuery object.  It returns the results with the keyword that was used tagged so we can highlight it and show the user where their search term was found within the document, that's great.  However, we need to further filter the results on a custom field that we have created.  It is easy to do that filtering with Camlquery by using the <FieldRef> tag to include what ever field we want to filter on.  But, the KeywordQuery approach does not use Camlquery, unless there is way that I am not aware of.    My question is, how can we do a keyword search and filter on a custom field that we have created?

Thanks,

Jeff