It can but that's working against the design. How you are "supposed to" do it is write reusable standalone objects in C (think the old CORBA way of doing things) and then write a TCL script as the main skeleton of your program that calls them when needed.
And now that I think of it I think the really orthodox way to do it was to write your whole program in TCL, then profile it and if you needed performance improvements use C to implement some of your more expensive blocks. The TCL library for C let you compile a shared library that the TCL interpreter would load and add the functions you wrote in C as new commands to the language.
