In this paper, we describe an approach for extending distributed system specifications. These specifications are structured as a parallel composition of subsystem specifications. The approach consists of building a new specification Snew by adding a new behavior described by a specification Sadded to a specification Sold with preservation of the properties of Sold and Sadded as well as the structure of Sold. Snew has all the properties of Sold and Sadded, if Snew can perform whatever Sold (and Sadded) can perform, and it does not block where Sold (or Sadded) does not block. We apply our approach for extending the functionality of a basic Automatic Teller Machine.