You can enable custom activity entities to support quick create forms, and you can create quick create forms for those entities. However, the quick create form for custom activity entities will not be used when people click the Create button on the nav bar. These quick create forms can be used only when people add a new record for a subgrid that displays that specific custom activity entity.
Source: https://technet.microsoft.com/en-us/library/dn531126.aspx#BKMK_QuickCreateFormEntities