Frankly its not likely to happen:
Liberia closed their intercountry adoption programme in 2009
Guinea has to be done through an approved adoption agency - there are none in the USA (If the USA don't have one then there isn;t one). I can only find evidence of 1 intercountry adoption USA-Guinea which I suspect was probably a family adoption.
Sierra Leone has (as I think Lilka says) a 6 month residency requirement although it can sometimes be waived. If you are prepared to do that - and to be honest if you plan to remove older traumatised children from their culture and community then I think you should be prepared to stay there some time with them - then you need to find yourself an attorney in Freetown as there are no adoption agencies in Sierra Leone. Sierra Leone is a non-Hague country so you will need to be approved here first then get approval over there then come back with them and re-adopt them here. All the while you will have social services involvement until they are re-adopted formally here - generally a visit about every 3- 6 weeks which is manageable though irritating. On the grapevine I hear that you will be expected to pay maintenance for the children from the time to are matched at a rate of several hundred pounds a month and that some orphanages aren't past stringing out the process in order to get some much needed funds. One american family who were trying to adopt two children from SL in 2012 estimated that the total cost was $21,000 (though that includes travel and accomodation costs).
The adoption "industry" doesn't have a good rep in SL - it was closed for investigation in 2009 as adoptive parents thought they were adopting "orphans" but they were in fact children with live parents who thought their child was being educated and fed elsewhere. The programme reopened in 2012 but it would make me nervous.
The UN is not keen on intercountry adoption post natural disasters as the children are often traumatised and removing them from their culture and country often exacerbates the trauma. Added to which government agencies have better things to do with their time than process the reams of paperwork that comes with inter-country adoption. It may sound tough for the individuals but in times of crisis governments need to focus
There is a good facebook group on children with traumatised pasts - Beta Beyond Trauma and Attachment which includes parents of other african children adopted after in-country trauma. You should read it and make sure you know what you're likely to be dealing with
I have one boy (adopted very small - pre-verbal) who suffered mild trauma and he's my only child and it can be hard going coping with a child post-trauma on your own.
How you cope on your own with a "family" of older children who don't speak a common language with you and don't understand your culture, when they are totally bonded to each other and not at all to you I have no idea.
It took me 3 years from application to bringing DS home from a country with about 5 previous adoptions to the UK over the previous 10 years (ie very few) but with plenty of information available from american agencies. I know of at least one adoption SL to USA which took that long so I think thats the kind of time scale you're talking about.
It took every ounce of perseverance I had to stick it out that long.
I spent my 3 years learning Russian which was invaluable. With an older age group I would say its almost essential that you learn some Krio. You could contact the embassy in London to find out the adoption procedures and where you might be able to learn some Krio.
If you are determined enough.