This is possible already, but may require a slightly different architecture to your drip campaigns, depending on how you already have them set up.
If you simply have one smart campaign running the flow of the entire drip (aka send email, wait, send email 2, wait, send email 3, etc) then it isn't easily doable to add someone mid step.
However, if you have each email separated out into their own smart campaigns, and drip members are passed from email to email via the "Request campaign" trigger and flowstep, you could potentially insert logic into certain email campaigns to accept new members from sources other than the last email that was sent. So in essence, your smart campaigns and flowsteps would look like this:
Email #1 Smart Campaign
Smart List:
whatever filters and triggers qualify your leads to start on this drip
Flow Steps
Send: email #1
Wait 1 week
Request campaign: Email #2
Email #2 Smart Campaign
Smart List
Trigger: Campaign is Requested (source is Marketo Flow Action)
Flow Steps
Send email #2
Wait 1 week
Request campaign: Email #3
Email #3 Smart Campaign
Smart List
Trigger: Campaign is Requested (source is Marketo Flow Action)
Flow Steps
Send email #3
Wait 1 week
Request campaign: Email #4
In the situation above, for emails #2 and 3, the Trigger "campaign is requested" can be triggered by any smart campaign that specifically requests it in its flow steps. Obviously one of those is the smart campaign that sends the first email, which then triggers the second email, but you could build other smart campaigns that request the later email smart campaigns (via the flow step "request campaign" function).
Thanks for the responce, yes you can build ALL your nurtures this way :-(. But my suggestion would be to eliminate all that extra work. And with the new Nurutre Campaing coimng out it would be great to have that feature.