I want to use segwit! Do I need to create a new wallet?
I am running my offline machine armory wallet in this way "armory --force-enable-segwit --offline"
No, your current Armory wallet supports them, and you have the correct flags passed into the offline machine I am running my offline machine armory wallet in this way "armory --force-enable-segwit --offline"
You will, however, have to create new segwit addresses. In Armory these are P2SH-P2WPKH addresses.
File > Settings > Fee & Address Types
Alter "Preferred Receive Address Type" to P2SH-P2WPKH and any newly generated receive addresses will be segwit addresses by default. You can do this for "Change Address Type" too if you desire.
How I sign a segwit transaction on my offline machine?
The same as a normal/legacy transaction, but it of course must involve P2SH-P2WPKH input addresses.