Using SPUserToken
When you need to do something in a list or item, in SharePoint, with a specific user you can use the SPUserToken.
Here is an example:
try{
SPWeb webOrigUser = SPContext.Current.Web;
//get the token for the impersonation user
SPUserToken token = webOrigUser.AllUsers[USERNAME].UserToken;
SPSite site = new SPSite(SiteId/SiteUrl, token);
.....
}catch{}