How can I include seperate Slicing Parameters in my 3mf file
Huh. On a hunch I just changed a copy of a .3mf model to have a .zip file extension, and what do you know, it works! 3mf is just a set of compressed xml. 

The zip folder structure I saw included a MetaData folder. I bet you could put just about anything you wanted in there, as long as it doesn't conflict with things other 3mf parsers are expecting to find.

I tested this. I dumped an unrelated pdf file into the folder and re-zipped it. Both MS 3D Builder and Cura were able to open the updated file just fine. However, saving the file again in 3D Builder did not preserve the PDF.

I suppose there's a lesson here, too: we may need to be wary of models we download off the internet. They could include literally anything.

This also raises an interesting possibility: password protected 3mf files. If 3mf is just a zip container, and zip files can have passwords, it follows logically that 3mf files can have passwords.