SPEM is a metamodel adopted by the OMG for software processes engineering. For the software process architectures description, SPEM architectural concepts are very insufficient. Indeed, the existing concepts disallow describing configurations and explicit links specific to software process architectures and finally their deployment. The objective of this paper is to present an extension of SPEM (System and Software Process Engineering Metamodel) with lacking architectural concepts. This extension is an important step to implement a new approach for software process reusing based on software architectures.