Other ways you could have but check with US Immigrant service as well as Nigeria Embassy:
1.) after 90 days expired, you could re-apply to extend another 90 days visiting visa. There is special form somewhere about extended stay on foreign country. You can have 6 months visiting visa request.
2.) Apply K1 Fiancee Visa application with other required US form in the US that allow to bring foreign spouse entering to USA much quicker than in Nigeria Embassy (i think), while you are down there in Nigeria to appy for Nigeria's settlement visa with your credible document and approved K1 Fiancee visa by US that will take 3 months if you follow exactly Nigeria visa procedure. K1 Visa is good for 90 days within getting married to foreign fiancee. If you make mistake that would delay up to 6 months, some one year, some more year to bring foreign husband entering to the US.
Read the previous thread here:
Going through the immigrant process to bring Nigerian husband to the UShttp://www.nairaland.com/nigeria/topic-63879.0.html3.) If you feel overwhelmed not understand all of this Fiancee VISA and visiting VISA procedure, hire your own experienced Immigrant lawyer in the US to help you. Their US retainer fee are varied between $300-$2,000 USD depend on what service they offer for you. Google it or use your local yellow page telephone book or lawyer referral service finding the right immigrant lawyer for you.
The cheaper US retainer fee does not include the immigrant application fee, you pay your own expenses for immigrant application fee, the highest US retainer fee include everything (legal advice, fully immigrant application submission, mailing service to send granted visa oversea embassy) to take care for you that you don't have to pay extra charge.
Do not use Nigeria lawyer down there due to rip you off stealing your money and run away!!! Unless if your Nigerian fiancee come from wealthy family, then great!!!!