The short answer is "yes."Another major concern in software planning is handling customer requests - that is, the requirements that arise *after* requirements gathering is done, but customers realise that they want something else added. My thought is to maintain a list of these requests; review and prioritize them at my weekly review, and then add them to the project plan as needed. Does this sound reasonable and flexible enough?
When items like this crop up during a development project, we track them in an "issues log." The log tracks when issues arise, to whom they are assigned, when they are resolved, and how they are resolved. "Resolved" means different things depending on the issue. Client questions are resolved when they are answered. Reported problems are resolved when they are diagnosed and a solution is either put in place or added to the project plan. New requirement are resolved when their impact on the project is assessed, a decision is made as to whether or not to add the requirement to the current release of the system, and the work of "retrofitting" the late requirement is added to the appropriate plan.
Adding late requirements as new sub-projects is really useful to show the client how much work the late requirements are causing. You will need this in order to renegotiate (or at least reset expectations about) the cost and duration of the development project.