I did this with a John Bull Best Bitter kit shortbrewed to 15 litres with 300g of sugar and 3 big tablespoons of black treacle.
After fermentation to make a Christmas Beer per gallon I added :
5g cascade, 4 tablespoons of marmalade, mixed spice, lemon zest, cinammon stick, allspice berry, 2 cardomom pods.
I just melted it down in water and simmered it for a few minutes and added it in. 12 days after bottling I gave it to people at a party and it was raved about. I'm going to do it again in the next few days.
The thing is with the marmalade you're really just adding a lot of sugar which won't bring any character itself, and with that whatever orange is leftover, which you could probably do more efficiently with other methods. Definitely do it if you've got loads - I'm doing it again because I picked up loads of jars of it cheap. I wouldn't bother with the Asda super cheap nonsense that's basically transparent.