In Prefs make sure you have Multiple Instances set.
Here I have 2 instances of Biab.
One has the original song I need a custom drum track for it's bpm is 120,
so in the second instance I load the original RealDrum wav or wma,
the closest bpm was 110 so I set the second instance to 110bpm,
I drag the track into the very start with Snap enabled,
I go through and find the sections I need then highlight and Copy.
I go to the first instance and paste it in the track but it's at the wrong bpm so I delete it then set the track bpm source audio to 110bpm,
now I paste and it will fit.
If you want hits from the end of the track you can take off of Snap then copy them then have it on Snap to paste, you have Paste (Mix) in the Edit menu or Ctrl+Shift+V this will mix the paste into the current section.

Watch: https://www.dropbox.com/s/npvpy7jpf20r713/BB23-Custom-Drums.mp4?dl=0
As I mentioned once we get non destructive editing all this will be a thing of the past.