First create a scoreboard objective:

/scoreboard objectives add selectMe dummy  

Then add all your Zombies:

/scoreboard players set @e[name=Name1] selectMe 1  
/scoreboard players set @e[name=Name2] selectMe 1  
/scoreboard players set @e[name=Name3] selectMe 1  

Now, you can execute your command:

/execute @e[score_selectMe=0] ~ ~ ~ summon ArmorStand ~ ~ ~ 

For more info, click here.

/execute @e[name=Unknown,type=Zombie] ~ ~ ~ /say hi

This command should work

1.13 Update

You can use the unless subcommand of /execute to execute as all zombies that don't have the custom name:

execute
  as @e[type=zombie]
  unless data entity @s CustomName
run ...