add and remove/pause leads in an engagement stream

A lead can only be in one Stream at a time. You would have to move the lead from A to D to achieve what you want.
Your Engagement should have defined Entry and Exit criteria. Normally I put the lead into Stream 1 first, but this is up to you. You could have a campaign pushing leads into Stream D if you want.

Hey Josh,

I am setting the engagement campaign up in a way that is very similar to a drip nurture. I am still opting for the engagement because of the advantages regarding the performance evaluation of content, analytics and the easier exchange/adding/removing of content, so the engagement would make our nurturing a bit more flexible.

I am not using the engagement campaign in its purest sense therefore, i.e. leads do not necessarily enter a stream, if they enter a stream (because of certain knock out criteria in the form, e.g. too young) it does not mean that they enter the next stream or any stream at all.

So that means that I need to be able to add and remove leads independently into and out of streams, without loosing the metrics though. I am able to add a lead to lets say stream A,using a smart campaign. I then use another smart campaign to remove the lead from the stream again (unfortunately I remove the lead then from the entire program which means I am loosing the metrics). If the lead should enter another stream I am just using a smart campaign again to add the lead to lets say stream D (knock out criteria for example not enough experience).

I have tried using "Change Program cadence" setting it to paused, but that resulted in the lead not being able to enter another stream because when I pause the lead in Stream A, the lead is completely paused and is not being pulled into stream D by the smart campaign.

So what's the best way here? 

Adrien,

To retain the data, drop paused leads into an Empty Stream. You won't lose the data and they won't receive content.

Managing Streams using external campaigns is fine...I guess I'd think harder about Drip vs. Engagement in this case: 
http://www.slideshare.net/jdavidhill/marketo-engagements-and-drip-nurturing-boston-user-group-dec-10-2013

Thanks Josh,

When I drop paused leads into an empty stream then I still have the issue that these paused leads cannot enter any other stream of the engagement campaign, correct?