Export Feed-Multiple line to one line

Hello, i want to make the multiple export lines to one line. We talk about export xml feed.I want to do this:
From this:
<size>S</size>
<size>M</size>
<size>L</size>
<size>XL</size>
To this:
<size>S,M,L,XL</size>

is that possible ?
I have Version 1.5.10 – WP All Export Pro