Squeezing in order to sweeten the fruit:
Important note: Regarding the definition of commonly squeezed fruits- See Halacha 2 and Q&A there!
By fruits that are not commonly squeezed for their juice even when in plentiful: If one is squeezing a fruit simply to sweeten the fruit and not for its juice then it is allowed [to be done according to all opinions] by all fruits other than strawberries, pomegranates and the like [of fruits which the custom of the world is to squeeze when they have plentiful of it, as will be explained below].
May one drink the squeezed juice? [Furthermore] even if one drinks the juice which is squeezed it is allowed being that one did not have intent to squeeze it for its juices.
The law by fruits which are commonly squeezed for their juice: However by strawberries and pomegranates and the like (of fruits which a segment of people which have a large amount of them squeeze them for their juice just like olives and grapes, and if there were to be this amount of these fruits by other people they too would commonly squeeze them for their juice) then [the Sages made it] forbidden to squeeze them even in order to merely sweeten [the fruit] due to a decree that one may come to also squeeze them for their juices (as do those people which have plenty of [this fruit]). [See Q&A if this ruling applies to all opinions]
[See Q&A with regards to inedible foods which will become edible due to the squeezing.]
Summary- Squeezing fruits in order to sweeten the fruit:
Edible fruits which are not commonly squeezed by the world for their juice even when possessing much of them, and are not squeezed in ones city [see Halacha 2 and Q&A there], are allowed according to all to be squeezed in all places in order to sweeten the fruit, even if one decides to also drink the juice. [Regarding fruits which are commonly squeezed-see Q&A]
Does the restriction against squeezing fruits which are commonly squeezed in order to sweeten them apply also according to the opinion which holds that fruits that are not commonly squeezed may be squeezed to drink their juice?
The Alter Rebbe brings this Halacha regarding squeezing to sweeten in continuation of the stringent opinion which holds that one may never squeeze any fruit to drink its juice. It is thus unclear according to Admur what the ruling in this would be according to the lenient opinion, which is the opinion that we rule according to. The Tzemach Tzedek however seems to side that this would be forbidden according to all.
Practically the following is the ruling of Rav Farkash: By grapes, olives, and all other fruits which are mainly grown for their juice it is forbidden according to all. By other fruits that are commonly squeezed to drink their juice but this is not their main purpose, one is to be stringent unless it is a case of need in which case one may squeeze it while having its liquid go to waste.
If a food will only become edible due the squeezing may it nevertheless be squeezed?
No, as this poses a Borer prohibition. This may not be done in any case, whether with intent to squeeze the liquid into food or to sweeten it.