Both STL and STEP are fairly common CAD/3D model formats and well supported in many programs, so it should be not hard to find one that can perform stl to step conversion. You can even find some dedicated, web based online converter that should help you convert stl to step format simply via your web browser.
Open STL file Open STEP file3D viewer and converter with huge file type support
A CAD viewer program with conversion capabilities
A set of 3D modeling tools
A converter for 3D formats
A CAD conversion software
Creo Parametric (formerly known as Pro/ENGINEER) is a complete 3D CAD software solution by PTC
A CAD visualizaton, analyzation and translating system
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
A dedicated CAD converter