How to add manually downloaded files to WSUS and use it in workgroup isolated area.

No, you need to let WSUS download its own files and those can be pushed via WSUS by Configuring update registry values ons those workgroup computers

updates from wsus is a pull operation not a push operation. The simplest way is to create a registry file and merge the file

Step 1: Create a *.reg file (wsus-client.reg) containing this:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate] 
"AcceptTrustedPublisherCerts"=dword:00000001 
"ElevateNonAdmins"=dword:00000001 
"TargetGroup"="Workstations" 
"TargetGroupEnabled"=dword:00000000 
"WUServer"="http://your-WSUS-server:port" 
"WUStatusServer"="http://your-WSUS-server:port"

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU] 
"AUOptions"=dword:00000004 
"AUPowerManagement"=dword:00000001 
"AutoInstallMinorUpdates"=dword:00000001 
"DetectionFrequency"=dword:0000000a 
"DetectionFrequencyEnabled"=dword:00000001 
"IncludeRecommendedUpdates"=dword:00000001 
"NoAUAsDefaultShutdownOption"=dword:00000001 
"NoAUShutdownOption"=dword:00000001 
"NoAutoRebootWithLoggedOnUsers"=dword:00000001 
"NoAutoUpdate"=dword:00000000 
"RebootRelaunchTimeout"=dword:0000000a 
"RebootRelaunchTimeoutEnabled"=dword:00000001 
"RescheduleWaitTime"=dword:0000000a 
"RescheduleWaitTimeEnabled"=dword:00000001 
"ScheduledInstallDay"=dword:00000000 
"ScheduledInstallTime"=dword:00000003 
"UseWUServer"=dword:00000001

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Step 2: Edit the lines:
-

 "WUServer"="http://your-WSUS-server:port"; and 
- "WUStatusServer"="http://your-WSUS-server:port"; 
to match the IP address (or FQDN) of your WSUS server. IMPORTANT: remove the ";" from the end of that lines!

Examples: 
"WUServer"=" http://WSUS.company.com:81 " 
"WUStatusServer"=" http://WSUS.company.com:81 "

"WUServer"=" http://192.168.0.1 " 
"WUStatusServer"=" http://192.168.0.1 "

"WUServer"=" http://intranet.local:8080 " 
"WUStatusServer"=" http://intranet.local:8080 "

The first key is named WUServer. This registry key holds a string value which should be entered as the WSUS server’s URL.

By default, in Windows Server 2012, WSUS 4.0 uses port 8530. However, WSUS 3.0 uses port 80, by default 
(How to Determine the Port Settings Used by WSUS - https://technet.microsoft.com/en-us/library/bb632477.aspx)

The other key that you will have to change is a string value named WUStatusServer. The idea behind this key is that the PC must report its status to a WSUS server so that the WSUS server knows which updates have been applied to the PC. The WUStatusServer key normally holds the exact same value as the WUServer key.

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Source: https://community.spiceworks.com/how_to/2267-deploy-wsus-to-clients-without-ad-domain-gp-using-the-registry

See the source for a remove-wsus.reg and further reading

There is a way to integrate existing downloaded packages into WSUS, but it is tedious, and for MS Windows Updates just not worth the effort.

Set up WSUS so it does not approve updates automatically,
apply to all clients what a group policy would do in a domain (the stuff David posted), with some changes as you like,
and  wait for at least two days, until enough machines have reported to WSUS.
Then approve what you want to get installed.
Machines being online should get the updates within the next two days after the update has been completed downloading.

You can try to force clients to report/check/download updates "immediately", but only on the machine itself:

net stop wuauserv
net start wuauserv
REM pre-W10:
wuauclt /detectnow
REM W10:
UsoClient /StartScan

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This doesn't work all the time, though.