There are actually 3 ways to do this
1) [% INSERT "whatever.txt" %]
Inserts the contents of whatever.txt without doing any template substitutions on it
2) [% INCLUDE "whatever.tt" %]
This processes the template and inserts it. Variables are localized (i.e. changes to variables in whatever.tt are not reflected in the parent template)
3) [% PROCESS "whatever.tt" %]
Same as above but variables are not localized. (i.e. changes to varibles in whatever.tt are reflected in the parent template)

More information can be found here here