So, when I create (or add RAID disks) I have this handy sfdisk template (I created once when I first added the 2TB disks):

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root:(charon.ka.heimdaheim.de) PWD:~
Wed Jul 09, 15:13:04 [0] > sfdisk -d /dev/sdj > ~/raid-disk-template.sf
root:(charon.ka.heimdaheim.de) PWD:~
Wed Jul 09, 15:13:04 [0] > cat ~/raid-disk-template.sf
# partition table of /dev/sdj
unit: sectors
/dev/sdj1 : start= 2048, size=3906793472, Id=da

So, if I wanted to add more disks, I’d just have to run the following:

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root:(charon.ka.heimdaheim.de) PWD:~
Wed Jul 09, 15:13:04 [0] > cat ~/raid-disk-template.sf | \
  sfdisk --force /dev/sdp