Can I use a 700c suspension fork on a 27.5" frame?
Maybe, we need to know more about the new bike.  

Wheel size - the 27.5" wheel will probably fit, (provided its not a plus or fat), but if its rim brakes, the brake bosses will be in the wrong place. You may or may not be able to find brakes that fit. If Disc brakes it will be fine

Axle - traditionally axles were all 9mm QR, now there are though axles so you will need to make sure the axle is the same 

Headset - standards have changed. Anything earlier than about 2010 will be straight 1 1/8", the tapered steerer became available and more and more popular on MTB's and is moving to others. 

Is it worth it - probably not. Hybrids don't typically come with great suspension forks, I would not recommend reusing an old fork unless it was a high quality one and the swap was easy (i.e. headset, axle, brakes compatable). Over time the forks degrade, especially if not serviced, and don't perform as well as they used to. A cheap suspension fork that's degraded is probably worse than no suspension.