Can I create a column of nvarchar(MAX) using FluentMigrator?

You could create an extension method to wrap .AsString(Int32.MaxValue) within .AsMaxString()

e.g.

internal static class MigratorExtensions
{
    public static ICreateTableColumnOptionOrWithColumnSyntax AsMaxString(this ICreateTableColumnAsTypeSyntax createTableColumnAsTypeSyntax)
    {
        return createTableColumnAsTypeSyntax.AsString(int.MaxValue);
    }
}