Just curious, can a barrel from a longer P-Series fit and work in a P95?  For example, P89 barrel --> P95 slide and frame?

I ask partially because I can't find a pre-threaded extended barrel for the P95.  I've seen services that will thread a barrel for you, but it doesn't look like they sell you a new barrel, they only thread an extended one you already seemingly have.  It appears that the P95 is the stubbiest of the bunch, so if you can fit a P90 barrel (4.2 inches) in a P95 (3.6 inches) then that's basically an extended barrel that you can then get threaded.

Of course, part of it too is just out of curiosity to see if it can be done at all.  Similar to how a Glock 19 barrel can fit in a Glock 26: you could then just thread the stock length G19 barrel and it will be extended in a G26 slide/frame when you drop it in.  You can buy an extended and threaded G26 barrel already, but if you REALLY WANTED TO you could do it this way as well.

EDIT: Fixed the first question, I meant another P-series 9mm handgun.  I do NOT mean switching calibers with a barrel just to clarify.  Got the P89 and P90 mixed up, thought the P90 was 9mm but it's a 45

Unfortunately what you want to do isn't possible.
The lock-up and recoil assemblies are markedly different between the P-89 and P-95.

Here are links to their respective manuals:
P-89 manual
P-95 manual

If you page down through them you'll come to the exploded diagrams of both guns and the differences will be obvious.
Hope that helps!