This is quite easy. Even though you only get about half the adam you would from harvesting them Tenenbaum gives you gifts after every few sisters including some of the adam you missed out on.
In the long run I NEVER needed that much adam, even without harvesting any little sisters I got the Tonic Collector and the Maxed All Tracks achievement in the same playthrough. (though I did deal with all little sisters as well)
You do not need to deal with all the little sisters, just not harvest any. I would suggest saving before you hit the rescue button just in case you mess up. It may not happen, but wouldn't you hate having to go through the same part twice.
This will not unlock until the final cutscene is over (or that's when it did for me anyways.) So don't get to nervous if it doesn't unlock immediately. If it hasn't unlocked by the time you return to the title screen then I would worry.
EDIT: Thought it might be helpful to give my tips for killing the Big Daddies so here a link to my guide for dealing with all the little sistersSolution for Dealt with every Little Sister in BioShock (Xbox 360)