I think that it might be simpler to only use Word for that.
In fact, I think "mail merge" just do that:
http://office.microsoft.com/en-us/word-help/word-mail-merge-a-walk-through-the-process-HA001034920.aspx[
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If not, then recording and editing a macro in Word might be simpler and get the job done.
If not, then automation using C# 4.0 might be the next easiest solution...
They are many other possibilities like OpenXML SDK.