We test and currently support migration from one multisite install to another multisite install at the network level.
The Multi Site Tools Addon enables additional functionality for WordPress multisite which includes the ability to push a subsite to a single site install , pull a single site install into a subsite, and export a subsite as a single site install.
The additional functionality provided by the Multisite Tools Addon is currently only available on multisite installs which means that pushing a single site into a multisite subsite or pulling a subsite into a single site install are not currently supported.
Additionally, we do not currently support migrating a subsite into another arbitrary subsite though there is a workaround. Migrating between a subdirectory multisite install and a subdomain multisite install is also not supported.