The usage of UML in specific contexts (like real-time systems or process modelling) is specially appealing since it provides a standard modelling notation widely used by the software engineering community. However, such usage usually requires to tailor (extend) the UML metamodel. The standard extension mechanisms suffer from several expressiveness limitations. In this report we identify these limitations and we define a two-tiered methodology to construct standard metamodels as extensions of the UML metamodel. Specifically, we present a methodology to obtain a conservative extension of the UML 1.4 metamodel (as a particular case of any well-defined MOF-model instance) and another to transform an extended UML 1.4 metamodel into a UML profile. As a result, we are able to combine the expressiveness provided by the explicit extension with the standardization coming from the use of profiles, which allows also the usage of existing tools.