Please don't. It's not that it's plant based material that makes for good flux. It's the RESIN in the pine shavings that makes the flux work better.
You're not going to have any resin in walnut shell media.
Find some other way to dispose of your used walnut media, and stick with sappy pine shavings for flux.
Funny I asked the same thing on castboolits.com
Response was.....basically. why add a dirty flux to lead your trying to clean up.
Something that slipped right by me on the think 'n process.
Hell I have so much pine saw dust in my amateur woodshop I would be glad to be rid of it considering the garbage men won't take it.. Its too bad you're not near me.