Well whether you'd use "berries" or "tap" the tree for some sap like with birch sap or maple, there's the 3 main ways of flavouring a mead.
Add the flavour to primary and then ferment.
Add the flavour to secondary.
Add the flavour to tertiary/post ferment.
Now presuming that the berries or sap or whatever it is that you use, it's fair to point out that flavourings added to primary, can, depending on the yeast used, reduce the amount of flavour, as some of the aroma compounds are just blown straight out the airlock.
Putting it in secondary (taking fruit for a slightly more "normal" example), helps as it retains a lot more of the fruit flavour/aroma.
Or, by adding it tertiary/post ferment, normally adds the most amount of the fruit aroma/flavour.
The later suggestions often make for an "earlier drinkable" mead, but can mess with the percentage alcohol (tertialry/post will reduce, while secondary can increase some).
I would go with the last suggestion, as it's probably easier to work out how much/how long depending whether it's berries (or pine cones) or sap. Small tests, using tiny quantities, of weighed or measured sap/berries etc, would then be scalable. So you can add increasing amounts to get the flavour you want/like.
Oh, and if it's possible to add the flavour as a liquid, it's likely to be quicker as you wouldn't have to leave the test/batch etc, to steep/extract the flavour and you'd have some idea about the likely effect of the flavouring.
Dunno if that helps any......