Is there a way to make a spillway in the preform? Then just make sure that your liner pond is below the preform. Then your preform becomes an extension of your waterfall. Is that clear and doable?

I'm not sure what you want to do. Do you want to dig a second pond next to what you have now, cut away some of the preformed liner to connect them and then glue the EPDM liner to the preformed? That has little chance of working. But if that is your plan, just get an EPDM liner big enough to line your preformed and the new addition.

There is a post on here about using gorilla glue and gorilla tape to glue a liner to a preform that is part of a stream or biofall. Linkie below ...

Here is a link that might be useful: glueing liner to preform