As if this hadn‘t been explained already more than 20 times in these forums... The short answer is no. The long answer is that a kind of programmer IC (the famous MKL02/04) is integrated on each Teensy board. The firmware of this chip is the only Teensy component which is not open source. Since everything else is open source and hardware schematics are available, you are free to build your own Teensys and source the components wherever, but in order to get things working, you’ll need to buy the pre-programmed MKL Chips from PJRC.