Um...I don't really understand what you are getting at.  Do you want to know if a flyable aircraft you download can be used as an AI?  Or if an AI aircraft you can download can be made flyable?

You can use a flyable aircraft as AI, but the catch is that Flyable aircraft are modeled in much greater detail than AI ones.  This is done so that the computer doesn't sepnd a lot of time rendering something that you probably won't see too upclose.  So the short answer is, yes, but be careful to not overtax your system.

You must create (or modify) an AI 'traffic' file, which is basically a set of instruction that tells the sim what aircraft should fly, from where to where, and when.

This is done using a utility called  TTools 2.02, by Lee Swordy. You can download it HERE (FS 2004 Utilities).  While there you can also download Afcad 2.21, by Lee Swordy. Afcad is a program to modify airports, which you may have to do if an airport does not meet the requirements to handle your ai traffic.

Both utilities have very good help files, and if you read them thoroughly it will explain the process, and you should have few problems.

As mentioned in the TTools help file, there are several programs available that provide a more user friendly method to enter the information needed by TTools. Some of them are available on the same FS 2004 Utilities page(s).  Traffic Route Creator v1, by C. Trew, on page 3, is one I found works nicely (though I still prefer the 'manual' method using Notepad.)

Dave