With something like Kiwi fruit which has pollen but no nectar Feeding sugar inside the hive can stimulate them to breed which stimulates them to gather pollen. The majority of orchard trees produce both nectar and pollen and are attractive to bees. I would not recommend feeding sugar outside the hive as a way of increasing pollination. Bees will naturally spread out around an orchard for a kilometre or two. Stone fruit is very attractive to bees providing conditions are right and if they are not warm enough or too wet no amount of sugar is going to make any difference. You can occasionally run into problems where some varieties are more attractive to bees than others but the only way around that problem is to saturate the area with hives to the point where the bees have no choice but to work everything. Individual bees almost always work the same flower type so if there is a bee on a plum tree that B will only work plums and a bee on apricots will just work apricots.
The only thing feeding sugar outside the hive will do (apart from causing a robbing match) is to decrease the number of bees actively foraging for nectar. Bees pollinate flowers whether they are gathering nectar, pollen or both.
I have seen attempts at things like spraying the trees with sugar syrup and that will undoubtedly increase the number of bees in the tree but they are not pollinate the flowers. Different trees produce nectar at different times of the day and so you need to check trees over the entire day to see if they are being visited. Given enough hives and warm dry weather over pollination is far more normal for stone fruit crops. Hope this is helpful.
external feeding is a cheap nasty way of feeding. i know guys who have done it. theres usually a ton of dead bees left behind.
in an orchard situation you will attract bees from neighboring orchards and it will turn into a robbing frenzy. they then can go beat up any hives near by, which is probably yours and rob out anything thats weak.
also if they get wound up they can end up stinging the crap out of any orchard workers/owners that come near.
as mentioned above, bees will do the job just fine without any help. the causes of poor pollination are typically out side of your control ie weather.