I would do it like this:
When you learn a daily power, pick an encounter power as well.
At the start of each day you can either have the encounter power or the daily power*.
For instance:
Bob the fighter gets to level 5, he selects Villain's Nightmare as his daily power and Steel Monsoon as a special encounter power.
At the start of the day, Bob the fighter specifies if he wants to have Villain's Nightmare OR Steel Monsoon available. If he has selected Steel Monsoon he can use it like any other encounter power, but he can't use Villain's nightmare that day.
The normal rule of not being able to pick a power twice would apply.
I think this makes a character more versatile and therefore slightly more powerful than usual, so you might say you need to take a feat, "Power versatility", each time you want to learn a daily power this way.
*For wizards the get their normal pick of 2 (or 3 if you have the feat) daily spells and one encounter spell. At the start of each day you either pick the encounter spell or one of the 2 (or 3) daily spells.