I can I add some good quality natural apple juice which I have purchased to the apple juice from my own trees when making cider? If so when shoudl it be added, before or after fermentation?
Make sure that your purchased juice does not contain Sorbate or Sulfites in the ingredients, if possible, because that can cause problems with fermentation.
If not possible, there is a solution:
Sulfites will "evaporate" after a few hours if left in contact with air, so let the juice breathe for a few hours before adding it to your batch; most sulfites will be gone.
Sorbate usually is used prevent the start of fermentation (it will not stop a vigourous fermentation), so if you start the fermentation of your batch, you may add juice containing Sorbate after a day of fermentation to your batch without harm.
If your purchased juice contains sulfites/sorbates, wait to have a vigorous fermentation before adding it to your batch.
If not, you may add the juice before or during fermentation.