How to map SOAP IDOC for specific Text TDID with multiple TDLINES into one Target XML field

I have a requirement where for a specific E1EDKT1 TDID value, I need to concatenate the multiple E1EDKT2 TDLINEs into one Target field defined as a String. The target field is defined as:

<xsd:element minOccurs="0" maxOccurs="1" name="Comments" type="xsd:string" />

I need to select the correct set of TDLINES and either Loop thru them to concatenate them together, or concatenate them to the target field. I've looked as using a variable, but I'm not sure if this will work.

Sender data looks like:

    <E1EDKT1 SEGMENT="1">
      <TDID>0102</TDID>
      <TSSPRAS>E</TSSPRAS>
      <TSSPRAS_ISO>EN</TSSPRAS_ISO>
      <E1EDKT2 SEGMENT="1">
        <TDLINE>Testing Transport Info Text</TDLINE>
        <TDFORMAT>*</TDFORMAT>
      </E1EDKT2>
      <E1EDKT2 SEGMENT="1">
        <TDLINE>Multiple line text test</TDLINE>
        <TDFORMAT>*</TDFORMAT>
      </E1EDKT2>
    </E1EDKT1>
    <E1EDKT1 SEGMENT="1">
      <TDID>ZFGP</TDID>
      <TSSPRAS>E</TSSPRAS>
      <TSSPRAS_ISO>EN</TSSPRAS_ISO>
      <TDOBNAME>MATERIAL FREIGHT GROUP</TDOBNAME>
      <E1EDKT2 SEGMENT="1">
        <TDLINE>REF</TDLINE>
      </E1EDKT2>
    </E1EDKT1>

Only when TDID = 0102 the result should be:

<Comments>Testing Transport Info Text Multiple line text test</Comments>

I don't have experience with PI mapping so all of this is very new to me. Would someone be able to provide a solution or lead me to a blog that would help me determine how to accomplish this requirement?