You can get siemens 840d lathe post processor which has to be configured to your desired controller which is in this case SINUMERIK 802C.
1.You should be aware of the difference between 802C and 840d controller commands.I mean the G and Mcode will be more or less same only for some special functionality it may differ.
2.Once you get to know what changes you have to do for 840d to make it 802c,you can open a 840d lathe post through post builder or any of the software available with you and start configuring accordingly.
I would start with the Siemens controls group, but be aware that what is available in the control may (or may not) have been implemented by the machine tool builder
A quick Google search on "Siemens 840D" (and backing up a couple steps in the heirarchy) got me to here:
If you can't find anything explicit, there are support links off that page
Production: NX10.0.3.5 MP5 + patch/TC11.2
I'd rather be e-steemed than e-diseaseled