Ok, there are several issues here.
1. ATP flight school can not help in any way with the EASA conversion, I assume you already know this.
2. To fly in the US at a part 121 carrier, you need an ATP. When you graduate from ATP you will have a Commercial Pilot Certificate.
3. To fly in Europe at an airline, you only need a commercial pilot license with the ATPL theory completed.
4. In order to get the European commercial pilot license and the EASA ATPL exams, you will have to find an EASA approved school. There are a handful in the US, but the majority are in Europe. ATP flight school will not train you for the European ratings.
5. Do you have European citizenship or right to work in Europe?