Shared Folder

You can use the Trusted Catalog to use a shared folder for sideloading

Open File Explorer in the root folder (C:\temp).
Create a new subfolder called 'shared-catalog' (C:\temp\shared-catalog).
Right mouse click and select Properties
Click on the Sharing tab and press Share
The Network access dialog box appears
Add yourself and press Share. Press Done.
Open Excel, File > Options > Trust Center
Press Trust Center Settings and choose Trusted Add-in Catalogs
In the Catalog Url enter the full folder path (\\machinename\shared-catalog)
press Add Catalog, Tick "Show in Menu" and press OK
Close Excel.

Place your manifest.xml file into the shared folder

Open Excel and display the Insert Tab.
Click on My Add-ins and display the Shared Folder tab.
Select this add-in and press Add.

Office 365 Registry Keys

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ 

HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ { GUID } \ (Default) - REG_SZ (value not set) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ { GUID } \ Flags - REG_DWORD (1) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ { GUID } \ Id - REG_SZ ( {GUID} ) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ { GUID } \ Url - REG_SZ (\\machinename\folder) 

Office 2016 Registry Keys

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ 

HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ catalog01 \ (Default) - REG_SZ (value not set) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ catalog01 \ Flags - REG_DWORD (1) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ catalog01 \ Id - REG_SZ (catalog01) 
HKCU\Software\Microsoft\Office\16.0\wef\trustedcatalogs\ catalog01 \ Url - REG_SZ (\\machinename\folder) 

This is the entry for a default Shared Folder

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogfsdefault \ Flags - REG_DWORD (1) 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogfsdefault \ Id - REG_SZ (catalogFSDefault) 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogfsdefault \ Url - REG_SZ (\\machinename\folder) 

This is the entry for a default SharePoint Catalog Folder

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogspdefault \ Flags - REG_DWORD (1) 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogspdefault \ Id - REG_SZ (catalogSPDefault) 
HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ catalogspdefault \ Url - REG_SZ (\\machinename\folder) 

disableomexcatalogs

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ disableomexcatalogs - REG_DWORD (1) 

disableallcatalogs

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ disableallcatalogs - REG_DWORD (1) 

requireserververification

HKCU\Software\Policies\Microsoft\Office\16.0\wef\trustedcatalogs\ requireserververification - REG_DWORD (1)