I'm in two minds about it - the sugar is good, but opening the keg is bad. Will the sugar off-set the keg being opened? Probably not. Getting sugar in the keg at the same time as transferring beer definitely has benefits, but I don't think I'd be opening an already kegged beer to put sugar in.
Diminishing returns?
I have 2 ales conditioning that I feel hadn’t fully attenuated so I hope the yeast was still active and fermenting away in the keg.
The two lagers I’m pretty sure we’re finished but they are aging at 40’ and one is pretty high abv.
Will add sugar to future kegs