There's really no way around this that I know because even if you install other anti-malware that itself always has to update its current definitions set from online servers to be able to work, or at least be fully protected.
I've always thought about this myself when I've built a new PCs and installed an OS and there appears to be no simple solution to installing protection offline. Copying definition database files for something like Malwarebytes from another already protected system and installing them offline is apparently not possible.
However with the much maligned MSE (Microsoft Security Essentials) you can get definition updates which will install offline. MS do not promote this option actively but the download page is relatively easy to find.
With a Windows XP VM I used with MSE (host PC using AVAST) the gits turned off the XP MSE update servers (April 2016? no warning) you could not update the definitions automatically or on demand. I used to update it manually by downloading and installing the file on a weekly basis.
However in this case you still have a problem because you are strongly advised not to have both MSE and AVAST or any other AV running on the same PC. Whilst you can get protection installed offline when using MSE, AVAST still has to be active to update its definitions.
There is no obvious 'safe' solution. So bite the bullet, install AVAST offline and then update the definitions. If you're really paranoid do a full system scan afterwards. The chances of something attacking your PC during the initial install of AVAST's definitions is small.
