At the moment you can only copy audio from one instance to another using the Audio Edit window, you can copy & paste overwriting the existing audio section or Merge, this works good for creating the drum section you need by copying the drum hits at the end of the RealDrum file.
Maybe you can have "Scrap" like RealBand, this can save any copied section to file then loaded back at anytime.

Scrap Buffer

When you Copy or Cut a section of the song, the data is placed in a temporary storage area known as the scrap buffer. This is where data is held when you Copy and Paste. When the Undo feature is used it puts the scrap back into its original location. You can also save the scrap to use at another time.

To do this, use the normal procedure to Copy or Cut a section of the song. Then go to File | Scrap | Save Scrap. This opens the Save Scrap dialog. Save the section of scrap with a descriptive name as a RealBand .SEQ file. When you want to use the scrap, go to File | Scrap | Load Scrap to reload the saved .SEQ file into the scrap buffer.


File - Scrap - Save Scrap

This command works just like the File - Save As command, but saves the contents of the Scrap Buffer to a .MID or .SEQ file. This is useful for exporting part of your work.

Note: Audio tracks will be saved to scrap if saved as a .SEQ file. However, if you use the Load Scrap command to load the file back into scrap, the track types must be set to audio or the scrap won’t be pasted into the song. In other words, if you saved a .SEQ file and track 1, 3, and 5 are audio tracks, you must set tracks 1, 3, and 5 to audio before loading the scrap.


File - Scrap - Load Scrap

This command works the same way as the File - Open command, except that the .MID or .SEQ file is loaded into the Scrap Buffer. You may then Paste the contents of the scrap buffer anywhere within the song. This is useful for importing a file into the current song.

While the recommended way of importing audio is by importing from .WAV files, you can use the Load Scrap command to load a group of audio tracks into the scrap buffer, so that you can paste them into the current song. If the audio file format of the .SEQ file doesn’t match the file format of the current song, the tracks in the file you’re loading will be converted to match the audio file format of the current song.

Note: If loading a .SEQ file with both MIDI and audio tracks, it is probably a good idea to make sure that both files have the track types match each other in order to ensure that all the data will be Audio tracks will be saved to scrap if saved as pasted into the song when you use the Edit | Paste command.

If the existing song contains some blank tracks, then RealBand will automatically set the track types to the correct track type. This won’t be done until you use the Edit | Paste command if the scrap buffer contains data, and also if the corresponding tracks in the project are blank but aren’t already set to the correct track type .