The Petri net standard ISO/IEC 15909 comprises 3 parts. The first one defines the most used net types, the second an interchange format for these — both are published. The third part deals with Petri net extensions, in particular structuring mechanisms and the introduction of additional, more elaborate net types within the standard. This paper focuses on the latter issue: how should a new net type be added, while guaranteeing the compatibility with the current standard. The extension of Petri nets with static or dynamic priorities is studied, showing design choices to ensure the desired compatibility. The result is integrated within the standard companion tool, PNML Framework. Then, the approach is generalised so as to be used at a later stage for other Petri nets extensions.