Is it posible to import an existing model file -xxxxxx.mdl- so that it can be edited with Gmax?Regards, thanks in advance

No! GMax does not do any rendering on the file. The GMax file is exported to an external file called MakeMDL.exe which compile the data into a Visual model file (*.mdl).The MakeMDL.exe has no capability to input the *.mdl file do decompile it.