I guess there is no easy way to work around here. There are two options in my humble mind.
Option A, move system/data to one part of the disk; keep it; create RAID on the unused part; move all system/data to RAID; create new RAID on the rest of the disk; steps are like following:
1. Shrink LV, then shrink LVG; -- easy
2. Shrink PV, free up unused PE; -- Doable, with risk;
3. Shrink partition size; -- Doable, with bigger risk;
4. create new partition & new RAID device; -- easy;
5. copy system/data to RAID; --easy
6. create new RAID on the old part of disk; --easy
7. create new PV and add to VG -- easy
I tried this option and stop in step 2, no further with step 3. because I found it too trouble some. So I essentially turn to the Option B, which turn out to be easy & fast.
Option B, reinstall the system.
Hope this experience can be helpful for those who looking for similar solutions.