Hi Flanker and welcome!
When you say "load a repeating table from an xml file" it implies you want to use a secondary data source (data connetions -> retrieve -> xml). Secondary data sources can't be saved. Only the main data source is persisted. However, you can copy the items from the secondary data source to the main data source and then save that back to the xml file. Set value works on single fields but not a repeating table. So, to copy a repeating table, the simplest approach is to use the Copy Table feature in qRules. (disclaimer: Qdabra Software pays for this free forum and also provides qRules trial for free or the full version for a small fee. The small fee helps us keep the forum running. ). The only other way to copy a repeating table from a secondary data source to the main data source is to use code.
To do this without a secondary data source is possible. You can create an XSLT that you use to merge the form into your main data source. The XSLT can copy repeating table data. Please look at the Status Report in InfoPath 2003 samples. It has built-in support to do this. Save the XSN as source files and look at the manifest.xsf. Search for import and look for the xsl specified. Open this xsl and view it. You can create a similar xsl for your form and use merge forms to do it. Users will open your form. Choose merge forms to import the xml file. Then save back to the same location manually.
I hope this helps.