You wish to allow your users to set a default signature within Outlook but also wish to deliver a signature using Signature 365.
The Signature 365 Outlook add-in sets the new and reply signatures in Outlook to None by default. This is done to ensure that signatures defined for the user in Signature 365 are correctly set, and will always remove a default signature set by the user.
It is possible to disable this behaviour and allow your end users to set a personal signature within Outlook. This will stop the add-in from adding your configured default signatures. It is not possible to both set an Outlook default signature and auto-insert a default signature from the add-in.
You can configure this by following the steps below:
Please note that this setting will affect your entire Signature 365 tenant and cannot be changed on a per-user basis.
Deselect the Manage Outlook's default signature check box.
Your users will now be able to set their default signature in Outlook.
You can remove access to all classic Outlook signature settings using the information in the article Managing personal signatures fonts and stationery for Outlook desktop.
You can remove access to signatures for devices using roaming signatures (New Outlook / OWA) by using the information in the article Disabling Outlook signatures in New Outlook and OWA.