I am a UK trained nurse. I have a social security number which means I can apply to the CA state board for application to test. When I apply, they will send me back a list of my deficiencies which will most likely be paeds, obstetrics and perhaps mental health. I can then apply to make up these deficiencies at an approved school within CA state. Then reapply and eventually be granted permission to sit NCLEX in CA. Then I must find an employer to sponsor me or use an agency.
If you don't have an SSN you can not apply to CA. But you can apply to other states. Other states aren't so strict about making up requirements like paeds, MH etc. It is obviously not possible to do 4 branches of nursing training in the UK.
You can figure out the hours requirements for each state board by visiting their website. Figure out what our school offers and then talk to your tutors about making up extra hours in each field if they are willing to let you do that. Also opt for spoke placements in places like NICU, gynae, do extra mental health hours etc. Its definitely possible, just a lot of work. If its something you have your heart set on you will do it 🙂 Good luck xx