Instead of putting keywords in the main file for the meta content tag, I want to create a separate file only for keywords and then refere to it in the index.html file. How can I do that?

See http://htmlhelp.com/faq/html/design.html#include-file . But I wouldn't bother. It's long ago Meta keywords were of any importance. The odd search engine may still use them, but I doubt any SE gives them much weight.

Please see the FAQ entry How do I include one file in another? for ways to include the same meta keywords element in every page automatically.

But see also Death Of A Meta Tag.