As Microsoft still doesn’t incorporate the default Outlook signature when creating mails via MAPI, see https://support.microsoft.com/en-us/kb/2544665, it would be great, if you could enable it when creating the e-mail.
All availble signatures and the default one you can find in %userprofile%AppDataRoamingMicrosoftSignatures and the registry. Here is a KB-article which describes some registry settings: https://support.microsoft.com/en-us/kb/2691977.
Also read this article, it describes another location where you can get the settings of the default signature: https://ifnotisnull.wordpress.com/automated-outlook-signatures-vbscript/configuring-outlook-for-the-signatures-within-the-users-registry/.
On the backlog for LL25. We’ll probably additionally offer a setting for the path to the signature, skipping the question of finding it in the registry if given. That way, you’re not restricted to Outlook signatures but can use an arbitrary HTML/RTF/Text file.