Enroll user's certificates for another user on the same machine

You can do the following:

• Start PowerShell as Administrator
• Do Enter-PSSession localhost -Credential (Get-Credential) and enter the credentials of the user that you want to add certificates on. (This may need a set up WinRM Service, use Enable-PSRemoting if you haven't have a running WinRM Service)
• Now you're in a PowerShell Session as the other user and can use Import-Certificate and Import-PfxCertificate etc. to add your Certificates for that user.

You can Check if it works by cd-ing and gci-ing around in the Cert: PSProvider after you imported the certs, they should be listed there. here's an example:

[localhost]: PS C:\Users\adminsystem\Documents> cd Cert:
[localhost]: PS Cert:\> cd CurrentUser
[localhost]: PS Cert:\CurrentUser> cd My
[localhost]: PS Cert:\CurrentUser\My> gci


   PSParentPath: Microsoft.PowerShell.Security\Certificate::CurrentUser\My

Thumbprint                                Subject
----------                                -------
F0BD97B4EC6CD8B71C35631738259CF9F2E54381  CN=Adobe Content Certificate 10-5, OU=Cloud Technology, O=Adobe Systems, L=San Jose, S=California, C=US
D1DF7F06B769BCCB3F4479041EC1F06E9CD3CB1A  CN=Adobe Intermediate CA 10-3, OU=Cloud Technology, O=Adobe Systems, L=San Jose, S=California, C=US

or, instead of entering a PSSession (even though I feel this is very comfortable for this task), you can do it directly with Invoke-Command (from an elevated PowerShell)

Invoke-Command -ComputerName localhost { Import-Certificate ... } -Credential (Get-Credential)

The third way - and the only way that works as a normal user is to simply start a new PowerShell as another user

start powershell -credential (Get-Credential)

and then do the cert import there.