It's possible to use a btrfs pool using the unassigned devices plugin, it won't look pretty since it's not officially supported (ie, only one device shows as mounted) but it works with any raid profile:
btrfs fi show /mnt/disks/Test
Label: none uuid: 75c7d7f5-74e4-4662-b465-c400b7384a6c
Total devices 2 FS bytes used 1.75GiB
devid 1 size 232.89GiB used 2.03GiB path /dev/sdf1
devid 3 size 232.89GiB used 2.03GiB path /dev/sde1
btrfs fi df /mnt/disks/Test
Data, RAID0: total=2.00GiB, used=1.75GiB
System, RAID1: total=32.00MiB, used=16.00KiB
Metadata, RAID1: total=1.00GiB, used=2.02MiB
GlobalReserve, single: total=16.00MiB, used=0.00B
ETA: You'll need to manually create the pool first (or use the cache slots).