You can combine repeating section fields in this way using a 'double eval' method.
Here's my little 'recipe'
xdMath:Eval(xdMath:Eval(<your repeating group>, 'concat(<field in repeating group to combine>,"<string or field to insert between on each combine>")'), "..")
Here's an actual example where the repeating group is called 'repeatingGroup' and the field is called 'field1' and the string between each combine is ";"
xdMath:Eval(xdMath:Eval(my:repeatingGroup, 'concat(my:field1, ";")'), "..")
and attached is a sample form which demonstrates it working.
Something of note is that you can't represent a carriage return in the above formula (ie what if I wanted to combine the fields with a carriage return between each one instead of a semi colon?).
The workaround I've used in the attached sample is a trick using a secondary datasource XML file with a field containing the special characters for a carriage return in an XML file. Then I use that field as the <field to insert between on each combine> and this gives the desired result.