Can I cut a hole and add a drain or pipe through the RPE Liner?

Yes, all our liners can be cut to install drains, skimmers, or other types of liner penetrations. The easiest and least expensive way to add a drain is with a bulkhead fitting - https://www.everything-ponds.com/bulkhead-fitting-threaded.html

Alternatively,

To attach a pipe to the drain, we would suggest using a bulkhead fitting along with an MPT to Slip Adapter, and then glue the pipe into the slip fitting with a 100% silicone that is designed for aquarium environments. Male Pipe Thread Adapter MPT X SLIP