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system drive encryption question


I am not currently using debian as linux OS but
considering it ...

If I clean install debian (latest of course) and during
the install process have  its / (system drive)
encrypted with pass-phrase ....

then later on, can I add a key, residing on
a usb flash drive,  to that encryption?

if yes, is there a step-by-step method one can follow  to do that?

thank you,