I am working as engineer in aerosport club, we have a olympic swimming pool, we need to reduce the depth for sometimes for recreational use. Is it possible and how?

Reduce only sometimes? Sounds pretty difficult to me. Hydraulic cylinders and a false bottom?

Why not just lower the water level?

Why not just lower the water level?

That's too easy an answer! LOL. I was thinking about just the deep end.

If lowering the water level isn't an option for some reason:
Custom sunken trampoline with no gaps on edges, situated at one end of the pool only?
(Do olympic pools have a deep end?)

Diving pools do and some are built for multi purpose, you start in the shallow end.