is it possible to add a virtual wireless interface in virtual openwrt?

nobody?

no, it is not possible to add wireless interfaces in virtual machines.

is still not possible to associate a wireless interface with a virtual interface of a virtual machine?

no it is not and such feature is not planned.

KVM in routerOS is not the newest relese and virtualized hardware might be not on par with drivers in JunOS. There is other interfaces available, you can try those.