Convert XML message to HL7 message

Hi Communtiy,
I am a begineer who is working and trying to understand Intersystems. I am developing an Ensemble interface which receives a XML message and converts it into HL7 format without using enslib.HL7 classes. 
Requirement

1. My file consists of patient details which needs to be updated in a sql table whenever the same patient arrives. We need to check if the patient exists and convert it into a pId segment.
2. we have encounter, observation details and need to convert these in HL7 ADT ^A01 message, please find the xml example below

<patient>
 <firstname>John</firstname>
 <givenname>Doe</givenname>
 <age>40</age>
 <mrn assigningAuthority="HospitalSystem">MRN12345</mrn>
 <address>
   <street>123 Main Street</street>
   <city>New york</city>
   <state>New york</state>
 </address>
</patient> 
<encounter>
 <encounterNumber>1234</encounterNumber>
 <type>Inpatient</type>
</encounter> 
<observation>
 <type>BloodPressure</type>
 <value>120/80</value>
 <unit>mmHg</unit>
</observation> 
<observation>
 <type>Temperature</type>
 <value>98.6</value>
 <unit>F</unit>
</observation>

let me know how to convert this message.