You cannot star items from Inbox, and you cannot pin items from Gmail. But you can find one from the other. After you find a list of them, it's easy to star all of the pinned messages or pin all of the starred messages (select all, do action).
You just need to search using "label:pinned" or "in:starred". Here are some examples of my searches.
Note: While you can label conversations in Inbox by clicking on the 3 vertical dots to the right of a conversation and using the "Move to" feature, "Starred" is not among the options for labels.
As for the other direction, I tried adding the "pinned" label to a message in normal Gmail but it looks like you have to create a new label that is in no way tied to the pinned status employed by Inbox.