What exactly are you looking to do? A document  library *is* a list
(just one with additional capabilities), so I'm not sure what you mean
by "import a list  into a document library"...

A client has a custom list  with hundreds of items in it. We are
interested in importing it to a preformatted document  library with a
Word template that contains a set of properties that are bound to the
library (so when either the Word doc  or the list is updated, the data
populates in both places).

The template I have is based on a document library. I don't know if
the same thing can be accomplished using the custom list.

Convert the list  to Excel & use Excel Services to keep the SharePoint version
and the Excel document  in synch. However, on the surface, it doesn't seem like
a wise thing to do. It's the equivalent of having "two master lists" rather
than a master and slave.

I might not be understanding the question, but couldn't you use a SPD workflow
to copy items from one list  to another if you have a slave and master type
setup?