Create domain user offline
Short answer: your workaround is fine, other than that: no.
Long answer:
It is not possible to create a user for a non existing domain, though the error message is somewhat strange:
CREATE USER [INVALIDNAME\SOMEUSER] FOR LOGIN [VALIDNAME\SOMEUSER] WITH DEFAULT_SCHEMA=[dbo]
GO
Msg 15006, Level 16, State 1, Line 5
'INVALIDNAME\SOMEUSER' is not a valid name because it contains invalid characters.


Running the same with &quot;VALIDNAME&quot; domain on both user and login side
works fine.

even if it would work, it would create a bunch of other issues for you (SID of INVALIDNAME\SOMEUSER not valid, mapped to wrong login, etc)


Your Workaround
With your workaround, on the other hand, it will be okay as the users are not touched when you restore the database on any server.
Another Workaround
Create a staging instance in the same domain as server B is, create all logins you need and create a database that contains all the users mapped to the logins you need.
Then, restore both, the master and your database, in a new instance running outside your domain.
I haven't tested this, but I am quite sure it should work.