One way is to play the game of giving land and then plotting to revoke something after you've pushed it, though this frequently means pissing off and going to war with the person you've just brought in. See under Revoke Vassal Title:
http://www.ckiiwiki.com/Plot
You can plot to revoke a county outside a duke's duchy. IIRC this means as a King you can invite a Duchy claimant, grant a County from your demesne, push and win the Duchy and then plot to revoke the County since its outside their duchy. You can then rinse and repeat giving this same county again to claimant after claimant, revoking each time.
They may reject your ultimatum to revoke the county and rebel. I believe (but am not 100%) that then means going to war (you get an option to back down here but I wouldn't) and when you win you automatically revoke that county as part of their surrender. I believe that since they're now classed as a rebel you can get one free-from-tyranny revocation on them which you can use to revoke the duchy that you just fought for (twice). Its now yours to give to whoever you want or keep.
There are also other ways to plot to revoke see that link.
EDIT: A less riskier way to do it is to give away a county in your own capital duchy as you can always plot to revoke any county in your personal capital duchy.