Really? I would have thought it would be easier in 4e than ever, because of its rules to level up (or down) monsters, and come up with stats based on level and role. It won't have a PC class, but it'll be a dragon with appropriate stats to the party's level.
In 5e, it'll be even easier--monsters have stats by level, and you can add PC class levels on top of that. So you have a dragon, which is a level 10 monster, and then you can have it gain levels and advance as a Sorcerer (or whatever).
You could take it as a feat.
A multiclass feat.
I aplogize, I should have been more clear. The 4e monster creation system is a direct response to the clustermess that was the 3e system and to a lesser extent, the 2e system. It is a very abstact way of dealing with things. Unfortunately. even with statistics for leveling up and down, it is hard to determine actual playable stats. The 4e formula assumes a role and feats you are not spending time muddling through. however, it does not tell you that number so you have to guesstimate it. Not to mention, from a cannon fodder DM stand point, according to the actual wording of the rules you cannot just give one of your orcs a set of +3 armor and merely add +3 plus armor.
Now, with 5e and it's current direction of "template" style backgrounds and specialties (and really - let's be overjoyed - the simplification of feats!) it will be less of an issue if they continue the course. But, for PC monsters under 4e, it requires quite a bit of DM houseruling.
But really the REAL culprit is the feat system introduced in 2e skills and powers and made canon 3e. All these minor benefits! A feat should be REAL advantages like class features! 5e looks like it is addressing this, though.