can i for example replace the partyID with the AccountNum from Custtable with an outbound port and AX standard ?
The standard value substitutions are just substitutions based on values not other fields.
If you are using AX2012 you can create an XSLT (translation) to do this job for you. From AX you will have an extended xsd and this can be translated to a specific one by using the xslt.
If you want to send the customers outbound, you can also include the Accountnum of the Custtable itself, but I think this was just an example?