Extending commercial tools for variability management is the problem addressed in this paper. IBM Rational DOORS is a well accepted tool in the area of requirements engineering and supports an Application Programming Interface (API) called DOORS extension Language (DXL). In this contribution we present a way of extending DOORS for product line requirements engineering based on a DXL plug-in called metadoc feature modeler. Established concepts in the community such as feature diagrams, instantiation of feature models, automatic deficit analysis and issue-based variability are introduced into DOORS. These concepts are abstracted using the DOORS meta-model. As DOORS is directly extended using new components, no special synchronizers are required between feature modeling environment and DOORS (which is a difference with respect to traditional feature modeling tools). We share our experience in using the feature modeler at a medium-sized company.