Both DXF and STP represent one of the most common CAD/3D formats and as such are support in a vast number of various CAD programs. Many CAD software should be capable of dxf to stp conversion just fine, you can even find some website based online converters for this job.
Open DXF file Open STP fileA CAD viewer program with conversion capabilities
A set of 3D modeling tools
A 3D modelling software
3D CAD/CAM tool for product development
Creo Parametric (formerly known as Pro/ENGINEER) is a complete 3D CAD software solution by PTC
A CAD converter with huge format support
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
A suite of CAD applications