You want to create your signature with custom and additional Azure AD attributes.
Is this possible?
However, you can include custom attributes, also known as extension attributes, within the list of directory extensions in Azure AD Connect when selecting up to 100 attributes for use with the Additional Azure AD Attributes feature.
Custom attributes vs Additional Azure AD attributes
How to access and enable Additional Azure Active Directory Attributes
How to enable and configure Directory Extensions in Azure Active Directory Connect
Which fields are multi-value and cannot be synced to Exclaimer Cloud?
Which Azure Active Directory attributes can be synced to Exclaimer Cloud?