asterisk users - Nov 2004 - if NOT SipUser then Dial(Zap/1/${EXTEN})

Hello

I like to setup a dial plan where sip users have the same number as 
there normal phone number.
So if sombody dial the number i like that asterisk looks first is it a 
number who exsist in MySQL sipfriends if not dial out with a ZAP device.

I know that extention.conf doesn't have a programming language but 
something like this

if (   SIP/${EXTEN}    )
{
exten => _XXXXXX,1,Dial(SIP/${EXTEN})
exten => _XXXXXX,2,Hangup
}
else
{
exten => _XXXXXX,1,Dial(Zap/1/${EXTEN})
exten => _XXXXXX,2,Hangup
}

Does anybody here a solution for this.

Thanks.


Sjaak