Add 2TB HDD + Fusion Drive to a 256GB ssd?

Hi all,

I'm about to purchase my first iMac so (my iPhone not taken into account) I'm fairly new with the apple platform.

The main goal wil be music production with Ableton live9 and 3rd party vst's with sample banks.

Therfore I want to get the best system possible preformance wise.

However, since I'm already way over my budget I want to start with the setup below containing the parts that (from what I undertsood) can't be changed later:

Hardware

4.0GHz quad-core Intel Core i7, Turbo Boost up to 4.2GHz

8GB 1867MHz DDR3 SDRAM - two 4GB

256GB Flash Storage

AMD Radeon R9 M390 with 2GB video memory

Magic Mouse 2

Magic Keyboard (British) & User’s Guide (English)

Accessory Kit

From what I understood I can add extra RAM later, just like I will add extra external storage for photos and movies etc.

The only thing that I´m still doubting about is which internal storage I should choose.

Apple offers 1 TB which comes with just 24GB.

I gues this is not enough for running Ableton vst's and sample banks.

However the 2 TB version is to expensive for now and 128Gb isn´t that much either.

For this reason I want to go for the 256GB version.

The only thing that is not clear to me if it's possible to add an extral internal HDD (7200 RPM) and create a fusion drive solution later?

This gives me time to save a little and eventually get the best of two worlds?

Thanks in advance and I look forward to hear from you,

Niels