I guess you meant "cold" wallet sitting on air-gapped device that was never been online and in the future will never go online. Some users are confused with two concept "off-line" device and "air-gapped" one and think if they turn off their WiFi or pulled Ethernet cable that would be enough.
Yes, I meant a "cold" wallet. The device will never go online.
1. From the online device take the unsigned file.
2. Sign it in the off-line device.
3. Go back and broadcast the singed file from the online device.