Multiple same name parameters

Hi,

For three days, I tried everything, but cannot make this work. Could You please provide solution to EXACTLY this problem:

https://wordpress.org/support/topic/multiple-attributes-with-the-same-name/ / https://wordpress.org/support/topic/xml-tags-in-prefix-and-suffix/

The provided solution with prefix does not work, it creates:

<PARAM_Veľkosť>EUR 40</PARAM_Veľkosť>
<PARAM_Farba>Biela</PARAM_Farba>
<PARAM_Materiál>Koža</PARAM_Materiál>
<PARAM_Štýl>Športový</PARAM_Štýl>

But this form is needed:

<PARAM>
<PARAM_NAME>veľkosť</PARAM_NAME>
<VAL>S</VAL>
</PARAM>
<PARAM>
<PARAM_NAME>farba</PARAM_NAME>
<VAL>biela</VAL>
</PARAM>
<PARAM>
<PARAM_NAME>Materiál</PARAM_NAME>
<VAL>80 % bavlna, 20 % elastan</VAL>
</PARAM>

I tried it with prefixes and suffixes, but it does not work at all then. The shopping agregator, Heureka.sk, for which this feed is, does not provide any help at all and are hesitant on the form.

Thank You.