You can ad an adapter but it can be harder to chase down any leaks, more fittings mean more connections means could have more leaks. You will need to make sure the adapters take flared lines. You can't just use some NPT fitting like for the cooling or fuel system, the brakes are much higher pressure over 600 psi so you have to use special adapters to stop leaks.
You may want to make sure you have the correct wheel cylinders or the correct brake line for your application before dealing with any adapters. Personally I would not use adapters in the brake system.
On a disc brake car there is one brass coupler on the passenger side that connects the hard line to the firewall line, it is designed to take double flairs.
As is the distribution block on the rear axle. The wheel cylinders except the flair from the hardline connection, so if you installed an adapter there it would need to take the flair from the hardline, and have a female inside the other side to go into the male cone in the cylinder.
If you simply install a regular step down adapter there it will blow brake fluid all over the first time to pump the system when the bleeders are closed.