Under file it should have "Write offline transaction" and "Put offline transaction online" or something along those words. Already has sign/verify message, sweet feature.
What are you talking about? The Bitcoin core wallet? Which wallet? If you want a wallet that does this, the Feature is already available for almost all wallet. (Sign message)
And if you want offline signing, the feature is already available as well;
https://bitcoinarmory.com/I think by "write offline transaction" he means there should be a feature in which we can make or submit offline transactions from bitcoin core and by "Put offline transaction online" he means that once the system gets online then submit or sync the transactions with the netwok.