Maybe I'm missing something here, but why not just do a search for the keyword and then replace it with the keyword and the additional text? If the Change field isn't sufficient, you can find the first instance of the keyword, add the additional text, then copy the whole thing and from then on search and replace using the By Pasting option.
ETA: Whoops, sorry. Just saw that you were looking for a scripting solution. I can't help with that.
This is a complicated task. Have you already located the keyword? If not, did you have a plan for how to do it? One way is with Search(), but it is not the only way. And a programmatic search is tricky.
I think we need more details. If you are starting from nothing, the best we can do is offer some pointers. We can't write the script for you, because it is not trivial. At least, speaking for myself.