Making GF Stout - want to add coffee for flavor - how much and when?

[Sematary]

So, since there is no dark barley malt in this recipe it will be missing the classic coffee/coca hints that would normally be there. I want to add coffee and dark chocolate to add these flavors. When would be a good time - while in 2ndary fermenter or maybe add during the steep or boil?
And how much for a 5 gallon batch? I was thinking 1/2 lb of each.