Waterbed 2 Matress Base?

Can I put a soft sided water bed straight onto an ensemble mattress base. My mattress has well passed it’s use by date but my base is still hard & firm.

I would not do that. I say no because the water bed mattress needs support all the way around. Think of it like a big bag full of water or in a sense, a big water balloon. There is a reason water beds have a full boxed in frame. When you get onto the mattress, the water displaces and pressure is distributed outward.