You can add, juice, fruit or sugar any time when fermentation is going on. You just have to be careful with the sugar to not put too much.
One more thing from my side. Grapes are perfect fruit for wine you just get the juice and you will make wine. With fruit wine is a little bit more complicate because you have to adjust sugar level and acid level. Blackberries contains 5-8% [lets say 6,5%] of sugar it means that using just fruit you will make very weak dry wine, around 4% of alcohol. It means that you have to add some sugar to make it stronger that yeast will produce more alcohol. I was reading that you are looking for dry wine and black berries continues 1% of citric acid what is high for dry wine. To reduce the acid level you have to add some water. You can always use only the juice but dry wine will be sour. Since it is your first wine just try and who knows it might be something you like. Add some sugar to the juice to make it strong enough and then you will see.
In a very short way I do it this way:
A*1,7*B=C
Where A the % level you are looking for in the wine B volume of the juice you have in liters, C the sugar you have to add to the wine.
Example:
I have 5 liters of juice and I want to make wine 14%
140*1,7*5=1190g
It means you will need to add 1190g of sugar into 5 liters to get 14% of dry wine. As I said this is the shortest way. It is not going to be exactly 14% since there is some sugar in the juice and adding sugar will increase the volume of the juice but for the begging it works fine. Just don’t put all sugar at one time! In your case I would just make it simple and make blackberry wine to see how all this process works.