You just add virtual-chassis config stanza to the existing switch (master) and then connect the new switch with QSFP cables.
set virtual-chassis preprovisioned set virtual-chassis member 0 role routing-engine set virtual-chassis member 0 serial-number xxxxx set virtual-chassis member 1 role line-card set virtual-chassis member 1 serial-number yyyyy
For your own peace of mind I would recommend you grab a copy of the config just in case something goes wrong.
Dont' forget to add interface assignments for the new member.
set interfaces ge-1/0/0 unit 0 family ethernet-switching set interfaces ge-1/0/1 unit 0 family ethernet-switching set interfaces ge-1/0/2 unit 0 family ethernet-switching set interfaces ge-1/0/3 unit 0 family ethernet-switching set interfaces ge-1/0/4 unit 0 family ethernet-switching set interfaces ge-1/0/5 unit 0 family ethernet-switching . . .