Andries There is no distinction at bunq between "main" and "sub" account. All bank accounts are called sub-accounts. They are all fully customizable, can be named whatever you want, have their own IBANs and are completely independent of each other.
The reason why bunq calls all the bank accounts "sub-accounts" is mostly for clarity so that users do not get them confused with their user account at bunq.
Short answer: yes you can do it with any account.
