Yes, you can add new steps (i.e. "shapes") to a Journey after it has been published. Once you've added the new shapes, re-publish the Journey and you'll see them on live view. Any contacts that enter the Journey after it has been re-published will then be eligible to flow into the new shapes.
For contacts already in the Journey, it will depend on the circumstances whether they will flow into the new shapes. This is explained further below.
Contacts who have ended their Journey won't flow into the new shapes
Let's say you have the following Journey:
Note: if you'd like a refresher on what the numbers mean, you can learn more here.
A contact submits the form, and flows into the Send Email action:
Because there are no shapes after the Send Email action, the contact ends their Journey.
Note: a contact could also end their Journey if they're not eligible to proceed; for example, if they flowed into a condition and didn't meet its criteria. You can use this technique to intentionally remove contacts from a Journey.
Once a contact ends their Journey, they will remain on the shape that they ended their Journey on, even if new shapes are added in subsequent publish.
So, for example, if you re-published the above Journey with these additional shapes, the contact would not flow into them:
If you want contacts who have ended their Journey to re-enter the Journey, you can do so by adding another trigger (e.g. a Smart Segment Trigger or List Trigger), connecting it to shape you'd like them to re-enter the Journey on, and re-publishing the Journey.
In this scenario you could use a Smart Segment Trigger:
Notice that I chose "Add all contacts in this segment". This only re-enters the contacts in the segment at the time of publishing the Journey. Contacts who submit the form in the future will enter the Journey from the beginning, via the Form Submitted trigger.
Contacts on delays that haven't expired yet are eligible to flow into the new shapes
Let's say you have the following Journey, where a contact has submitted the form, received the first email and is now on a one day delay:
Unlike other shapes, which are processed immediately, a contact will remain on a delay for the time period specified on the shape at the time they enter it. Because they haven't ended their Journey, you can add new steps later in the Journey and they will be eligible to flow into them.
For example, if you wanted to add a third nurture email in the example above, you could do so:
Note, however, that if a delay is the final step in your Journey, a contact will end their Journey when the delay expires.