It should be possible to connect Top Level Process Node with Start Event inside of Subprocess

According to specification:

A Start Event MUST NOT be a target for Sequence Flows; it MUST NOT have incoming Sequence Flows.
An exception to this is when a Start Event is used in an Expanded Sub-Process and is attached to the boundary of that Sub-Process. In this case, a Sequence Flow from the higher-level Process MAY connect to that Start Event in lieu of connecting to the actual boundary of the Sub-Process.

but it is not possible in current Engine implementation.

For more details see BPMN 2.0 Specification page 245.

Update: The same is valid for End Events, you should be able to connect subprocess' End Event with next node.