File Content Conversion From Nested XML to Flat File

I have as the output of my mapping:

<MT_INVOICE>   (1...1)

     <Header>    (0....unbounded)

          <HeaderField 1>

          <HeaderField 2>

          <HeaderField 3>

          <Item>   (0... unbounded)

               <ItemField 1>

               <ItemField 2>

               <ItemField 3>

My output needs to look like:

HeaderField 1, HeaderField 2, HeaderField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

.

.

.

HeaderField 1, HeaderField 2, HeaderField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

.

.

.

HeaderField 1, HeaderField 2, HeaderField 3

ItemField 1, ItemField 2, ItemField 3

ItemField 1, ItemField 2, ItemField 3

.

.

.

I understand basic File Content conversion but I am unable to generate any output. I keep getting a standard error message in RWB tellng me my content converion is wrong.

Is it possible to do this with standard FCC, and if not, what would be the correct path to do this?

Current FCC setup:

Recordset Structure   Header, Items

Header.fieldSeparator           ,

Header.endSeparator            , 'nl'

Header.fieldFixedLengths      3,8,3,9,9,10,13,14,13,42

Header.fixedLengthTooShortHandling    Cut

Header.fieldNames                 Bunch of names

Item.fieldSeparator                 ,

Item.endSeparator                  , 'nl'

Item.fixedLengthTooShortHandling     Cut

Item.fieldFixedLengths           3,8,3,9,9,10,13,14,13,42

Item.fieldNames                    Bunch of names different than above