First one would have to define "Female singing voice"

It's normal to immediately take this to mean high-pitched and if that's what you meant, then it is very difficult, but OCCASIONALLY can be done IF one is blessed with a larynx etc that can adapt.
There are a lot of famous and well respected female vocalists who have comparatively low pitch and these voices are often judged to be very sexy.
Surgery to raise pitch is very risky and the end result very unpredictable indeed - poor tonal quality or hoarseness and sometimes even total loss of voice.
Although pitch is undeniably an important factor in judging a voice male or female, it has been established that the critical threshold is approx 165Hz - about the open 3rd string on a guitar, "G" below Middle C. This is actually quite a low frequency that is well within the reach of the upper end of the male range.
Below 165Hz one is likely to be instantly judged male.
BUT - Staying above it does not make you sound female, there is a lot more to it than that - the length and size of various resonating parts of the neck and head, speech and stress patterns etc. Speech therapy addresses these issues, training you to keep the larynx held high(er) to raise pitch and avoid chest resonance, to "place" the voice so that one utilises more female sounding resonant parts and to change attack and inflection patterns.
Once testosterone enlarges your larynx etc there is no way back and the only way that 99%+ of males can get above the lower medium female range is by using Falsetto.
Few guys can produce a decent falsetto that is full and rounded in tone, it invariably sounds thin, scratchy, forced and patently false

- think Terry Jones in Monty Python drag sketches.
The problem for transwomen is one of Range. Ones range remains the same, so if the lower half is discarded then one doesn't have much scope left in an upward direction.
Bottom line - Can a transwoman -who can pitch and sing anyway - learn to produce a very pleasant and very convincing singing voice that sounds totally female - Yes, certainly.
- but she won't be singing "Ave Maria" at glass-shattering pitch!