The only way to add more space for holding program code is to use the QSPI flash controller and an external QSPI flash chip. The Nordic NRF52840 DK reference board includes an 8MB QSPI flash, and I'm pretty sure the Nordic SDK has an example in it somewhere that uses it. You need to enable eXecute In Place (XIP) mode so that the flash is mapped into the CPU's physical address space.
(Note: yes, it's actually an 8 megabyte QSPI flash chip. It says 64Mb in the specs, but the lower case 'b' means 64 megabits, which is equivalent to 8 megabytes. You can use a larger chip in your own design.)
