Importing and Exporting Models
The only programm to import and export model files from GTA V at the moment is ZModeler 3 (proprietary). The .yft file format is used for models, .ytd for texture archives.
ZModeler 3 provides basic modeling tools. However, any other program that allows exporting into a format that ZModeler 3 can import, such as .obj or .3ds, can be used. Many modelers use 3ds Max (proprietary), which has more advanced 3d creation tools compared to ZModeler.