Can I call a javascript function (from Classic ASP) from within my ASP.NET codebehind and obtain it's result?

Here's a way to wrap the server side JScript function using a simple local "web service" call.

Add a JScript or ASP script that references the hashing function:

File: DoHash.asp

<!-- #include file="passwordHasher.js" -->
<%
Dim password
password = Request.Form("password")
Response.Write HashPassword(password)

%>

In your ASP.NET application add this method (and class if necessary somewhere relevant:

public class HashWrapper
{
    public static string HashPasswordFromJScript(string password)
    {
      string url = "https://mysite.com/DoHash.asp";

      string fields = "password=" + password;
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
      request.ContentType = "application/x-www-form-urlencoded";
      request.Method = "POST";

      using(Stream rs = request.GetRequestStream())
      {
        using (MemoryStream ms = new MemoryStream())
        {
          using(BinaryWriter bw = new BinaryWriter(ms))
          {
            bw.Write(Encoding.UTF8.GetBytes(fields));
            ms.WriteTo(rs);
            ms.Flush();
          }
        }
      }

      using(HttpWebResponse response = (HttpWebResponse)request.GetResponse())
      {
        if(response.StatusCode == HttpStatusCode.OK)
        {
          using(Stream rs = response.GetResponseStream())
          {
            using (StreamReader rr = new StreamReader(rs))
            {
              StringBuilder sb = new StringBuilder();
              int bufferSize = 1024;
              char[] read = new Char[bufferSize];
              int count = rr.Read(read, 0, bufferSize);

              while (count > 0)
              {
                sb.Append(read, 0, count);
                count = rr.Read(read, 0, bufferSize);
              }
              return sb.ToString();
            }
          }
        }
        return null;
      }
    }
}

In the part of the application you need to hash a password just call:

string hashedPassword = HashWrapper.HashPasswordFromJScript(password);

It's not pretty but it'd get you by. I'm also presuming your site can encrypt traffic using SSL.