Incompletely tagged data

I'm new with machine learning so please bear with me. All pointers and suggestions are highly appreciated.

Suppose I want to assign part of speech tags to a text. How would I go about when some of the tags are already known, but only to a certain extent? Example:

Dogs^NOUN live^??? in^PREP houses^NOUN near^PREP humans^???

??? signifies an unknown tags. I'm aware that POS tagging typically doesn't use tags like these, but I hope using them illustrates my issue.