Finally I was able to get this done with help from my colleague and this link courtesy Malcolm Swaine at
http://www.codeproject.com/Articles/32600/Manually-validating-an-ASP-NET-user-account-with-a
Here is the final code that should do the trick.
uses DECHash,Data.Cloud.CloudAPI;
...
..
function GetHash_ASPNetMemberShip(const Password, Salt:string):string;
var
bPassword ,bSalt ,bSaltPassword: TBytes;
begin
bPassword:= TEncoding.Unicode.GetBytes(Password) ;
bSalt:= DecodeBytes64(Salt);
SetLength(bSaltPassword, length(bPassword)+length(bSalt));
Move(bSalt[0],bSaltPassword[0],length(bSalt));
Move(bPassword[0],bSaltPassword[length(bSalt)],length(bPassword));
Result := THash_SHA1.CalcBuffer(bSaltPassword[0], Length(bSaltPassword), TFormat_MIME64);
end;