This used to be possible in early versions, but is not supported in LoRaWAN; see below.
As far as I know, the command line ttnctl tool still allows for that. On one line:
ttnctl devices register personalized <your DevAddr> 2B7E151628AED2A6ABF7158809CF4F3C 2B7E151628AED2A6ABF7158809CF4F3C --relax-fcnt
However, these are the Semtech default demo keys, and are insecure as everyone knows them. Also TTN does not truly support those keys:
The default keys are not supported in the new backend because they are not secure.
[…]
We will not actively reject keys, but messages might not be delivered if someone else registers the same device address with the same keys.