Hi Daria,
Your requirement needs to be changed, then only you can adapt Project System Functionality.
You can consider by creating one project (the head office country) for the higher node and create one WBS element for each country and attach AUC and collect cost incurred at country level. You can use inter company transaction to post cost for the each WBS element (Country). and settle the cost from WBS element to AuC.
Since each country has its own Tax and Policy, so you cannot simple make the system work based on one country and spending money in different country.
Regards
Terence