Both the JT (.jt) and STEP (.stp, .step) CAD formats are well supported in various CAD/engineering software and as such it should be fairly easy to find a CAD software or CAD conversion/translation system that would be capable of jt to step conversion.
Open JT file Open STEP fileA CAD conversion software
Creo Parametric (formerly known as Pro/ENGINEER) is a complete 3D CAD software solution by PTC
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
A dedicated CAD converter