  Multi-Arm Multi-Stage (MAMS) platform trials are an efficient tool for the
comparison of several treatments. Suppose we wish to add a treatment to a trial
already in progress, to access the benefits of a MAMS design. How should this
be done?
  The MAMS framework requires pre-planned options for how the trial proceeds at
each stage in order to control the family-wise error rate. Thus, it is
difficult to make both planned and unplanned design modifications. The
conditional error approach is a tool that allows unplanned design modifications
while maintaining the overall error rate. In this work, we use the conditional
error approach to allow adding new arms to a MAMS trial in progress.
  We demonstrate the principles of incorporating additional hypotheses into the
testing structure. Using this framework, we show how to update the testing
procedure for a MAMS trial in progress to incorporate additional treatment
arms. Simulations illustrate the possible operating characteristics of such
procedures using a fixed rule for how and when the design modification is made.
