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Most modern contributions for flexible workflow management propose proprietary solutions or extensions of standard workflow execution engines to enable the required runtime control-flow deviations. This fact results from the widespread viewpoint that for the realization of deviations, manipulations should be performed directly on the concerned workflow model or instance. It is neglected that most contemporary workflow management systems already contain a very strong concept for runtime deviations, namely sub-processes which can be replaced with new versions at any time. In this paper, we show that execution flexibility concepts like variant construction, aspect orientation and runtime adaptation can be emulated by regular workflow models. For this purpose, we propose a generic automated design-time model transformation technique which extracts variant workflow parts to sub-processes, makes use of "adaptation processes" following a specific pattern syntax and generates a selector structure performing recursive sub-process calls supported by a rule engine. Its main benefit consists in a general reduction of implementation efforts for flexible workflow engines. For evaluation purposes, a reference implementation of our model transformation approach was realized in SAP Net Weaver BPM/BRM and produces fully BPMN2 compliant executable artifacts together with a rule set, which is used as the main interface for the specification of frequently changing process deviations.