By defautl, only the Title field is available for document upload. All other
fields are customizeable.

Go to the document library that you want to enable the keyword  field and use the
following steps:

1. In the Quick Launch Bar, select Modify settings and columns
2. Under the Columns section, select Add a new column
3. In the Column Name field, type in Keywords
4. Select either single line of text or multiple lines of text
5. Select whether this field is a required field or not
6. Click the OK button
7. This will added the Keyword field on the form for all documents that will be
uploaded to this particular document library

One other quick question about this. Will the portal search return
results from keyword  matches if I use the method described below?

You need to perform a Full update on your indexes. If it is a Document
Library located in the Portal you should hit the "Portal_content" index.
When the status changes to "idle" again go to the Site Settings Page and
Select "Manage Properties from crawled documents" in the "Search
Settings and Indexed Content" section.

Expand the tree node named "urn:schemas-microsoft-com:office:office" -
here you should be able to find your property. Click on it and change
the settings according to your needs (probably found in the section
"Search Options")

It might take a moment before the changes take affect - otherwise
perform an iisreset from the command prompt.