That would be nice to tanker fuel if you had the weight margins,but:
In loose terms on a heavy jet,
on burns 10% of the excess fuel for every 1000 nm that it is carried.
Eg,
nzaa to phnl 3800 nms :load 10000kgs of 'mum and the kids fuel,
you will arrive with only 6200kgs of the excess ..
Expensive isn't it