Install WP All Export.
Create a new export and select only the products.
In filtering options add the following rules:
Sku is not empty
Status equals publish
Price is not empty
In export type select Feed > Custom XML Feed and add something like this:
<?xml version="1.0" encoding="UTF-8"?>
<MPITEMS>
<products>
<!-- BEGIN LOOP -->
<product>
<category>{Κατηγορίες προϊόντων}</category>
<SKU>{Sku}</SKU>
<name>{Title}</name>
<EAN>agrofitiki-{Sku}</EAN>
<manufacturer>{Μάρκες}</manufacturer>
<description>{Excerpt}</description>
<url>{Permalink}</url>
<image>{Image Featured}</image>
<additional_image></additional_image>
<price>{Price}</price>
<quantity>{Stock}</quantity>
<shipping_lead_time>0</shipping_lead_time>
</product>
<!-- END LOOP -->
</products>
</MPITEMS>
Use this link in order to check the validity of your XML.