Yes, probably, but over 18 you move from "minor" adoption to "adult" adoption. Any family law specialist should be able to assist you. For background, see http://www.willicklawgroup.com/adoption.
Step parent adoption here in Nevada is fairly straightforward, and even more so when the adoptee is an adult.
Usually the greatest obstacle to step parent adoption is terminating or obtaining a relinquishment of the parent’s rights with whom you want to sever family ties. Because you are an adult, no rights need to be relinquished or terminated.
You and your prospective step mom will need to formally consent to the adoption, and then a Petition for Adoption can be filed. After the Petition is filed there will be a hearing in family court and the judge will sign the Adoption Decree. Unless you and your prospective step mother want to wait until after the wedding to complete the adoption, your step mother can adopt you at any time.