In this work, a design concept for micro-turbojet to micro-turboshaft engine conversion is presented. This is motivated by a lack of available micro-turboshaft engines which is shown in the market survey conducted. Thus, the presented concept deals with the conversion of an existing micro-turbojet engine to a micro-turboshaft engine for a specific power output. The conversion is shown using the micro-turbojet engine OLYMPUS HP from AMT Netherlands. Furthermore, the simultaneously developed analytical preliminary design of the additional single-stage power turbine is shown besides a thermodynamic cycle simulation. This has been done to obtain the unknown gas generator outlet condition which is similar to the power turbine’s inlet condition. Within the cycle calculation, occurring losses due to the small dimensions have also been considered. During the design process, different combinations of work coefficient and mean diameter of the power turbine were investigated to minimize the required gear box ratio for a given rotor speed in terms of weight minimization. To keep losses in the power turbine low, the preliminary blade row has finally been improved using CFD calculations.