In the context of Model Driven Development, models play a central role. Since models can nowadays be executed, they are used not only for description but also for production [32][30][24]. In the field of software process modelling, the current version of the OMG SPEM standard (ver1.1) has not yet reached the level required for the specification of executable models. The purpose of SPEM1.1 was limited at providing process descriptions to be read by humans and to be supported by tools, but not to be executed. Therefore, the OMG issued a new RFP in order to improve SPEM1.1 [35]. Since we intend to participate in the next major revision of SPEM, namely SPEM2.0, in this work, we: 1) compare SPEM1.1 both with primary process model elements (i.e. Activity, Product, Role,...) and with basic requirements that any Process Modelling Language should support (i.e. expressiveness, understandability, executability,...); 2) identify its major limitations and advantages and 3) propose a new UML2.0-based metamodel for software process modelling named: UML4SPM. It extends a subset of UML2.0 concepts – with no impact on the standard – in order to fit software process modelling.