We have a function in the toolkit called Get-UserProfiles which gets the path to all of the users profiles on the system and also the default profile. The default profile is used to provision the profile for newly created users to changes you make there will be replicated to new users.
Here is one example from the function showing how to use it:
## Returns the user profile path for each user on the system. This information can then be used to make modifications under the user profile on the filesystem. [string[]]$ProfilePaths = Get-UserProfiles | Select-Object -ExpandProperty 'ProfilePath'
Then just use a loop to cycle through the profile paths and make the appropriate file changes you want to make.