Tim,
No, Transactional replication itself does not alter your user table schemas. However, published tables MUST have a primary key, so in that sense, you might have to alter the tables yourself prior to replication.
Note, however, that (very) strictly speaking, enabling Transactional replication does change your database slightly by adding system tables such as syspublications and sysarticles.
Hope this helps,
Scott Thornburg