Both STL and STEP formats are fairly common CAD/3D model formats and well-supported in many programs. So you should have no trouble to find a program capable of stl to stp conversion. Many 3D modeling tools or converters can be used for it as it should not be difficult to actually convert stl to stp.
Open STL file Open STP 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