The answer is "sort of." Now, the below is part speculation, but its informed speculation. Details may be wrong, but the gist should be right.
Basically, the idea of adding class levels 3e style is out. Instead (in short), you look at the monster, as yourself, "what Wizard abilities do I want this Dragon to have?" (or whatever) and then you just give those abilities to the monster.
Let me give examples.
Suppose you're playing 3e, and you have a CR 5 monster called a Foo. You want to make a CR 7 Foo Wizard. So, you take the CR 5 Foo, and you add two levels of Wizard to it. If Wizard levels don't synergize right with the Foo's basic abilities (its a melee monster receiving wizard levels) you might add 3 Wizard levels, and still end up with a CR 7 monster. Based on those 3 Wizard levels, you have a selection of spells per day that you pick for your Foo Wizard.
Now suppose you're playing 4e, and you want to do the same thing. So, you take your Level 5 Foo, and you look at what "type" of monster it is. Suppose its a Soldier. You want it to be a Wizard though, and that's more of an Artillery monster. So you check a chart for baseline Level 7 Artillery monster stats, and combine those with the Foo's existing abilities. Then you assign it some Wizard abilities of your choice, directly from the Wizard's power selections.
The biggest difference is that the 3e system involved sort of adding together legos. The original monster was a big lego, and then you added two or three little "wizard" legos to get an overall monster who's size and shape was predetermined by what legos went into its creation.
In 4e, you start with your final concept, and make it so, with guidance from the DMG. The 3e process requires few judgment calls- if a Foo gains 3 levels of Wizard, it can cast spells available to a 3rd level Wizard. End of story. The 4e process is different- the Foo can cast any spell you want it to cast. Instead of telling you what spells and how many, the rules now give you general advice on how to know what's appropriate, and rely on you to do your best.