There are two ways to send money from BD to overseas.
1. If you know someone reliable, that's coming to your country (overseas), ask them to convert BDT into USD and bring it with them.
2. You'll find lots of people who send money to BD, just tell them to pay you USD and pay their families BDT in bangladesh. This way many people agrees as they don't have to pay transfer fee and will get better rate.
Hope that helps
please don't harass me by saying legal or illegal.