Is it possible to retrofit additional 'waveform memory' to increase the montage capacity beyond the current 1.8GB?

Not that I am aware of. I don't believe there are any 'physical' updates that can be done.

If not, what's the best workaround to play a click track or a backing track ?

I'm not sure I understand how that is related to the first question.

Can you provide more info about your system? Have you run out of user memory?

One way to free up user memory would be to move your 'archived' data to libraries. You can have 8 libraries loaded and each of them can store waveforms as well as performances and other object types.

That would free some user memory you could use for additional waveforms.

But it would help if you told us what limitations you are running into.

Use a different device for your backing track. Say like an iPad. Plug that thing into the USB TO HOST port and play your backing track from the iPad which has as much memory as you have in the iPad.