As you already aware that need to manually Click on Suspend button in Control Panel Bitlocker Drive Encryption in order to successfully inplace upgrade Bitlocker Computer to windows 10 without need to perform decrypt the Computers.
And Below are the powershell command line for Bitlocker for you to add in to Task sequence and
I've tested those command line is working fine.
To Suspend Bitlocker
Suspend-BitLocker -MountPoint "C:" -RebootCount 0
To Resume Back Bitlocker
Resume-BitLocker -MountPoint "C:"
Task Sequence Capture Screen at below:-
Suspend Bitlocker command line
Resume Bitlocker command line