This is an update to my post above. 

As shown in the ticket above, an update of the 'Currency code' setting was made by the UNA Team to add more choices. But only 4 currencies were added to the list of the default Currencies.

For those like me who wanted to add their new currency and get it displayed as a default system currency in Studio > Settings App > System > General > Currency code (for checkout system), should do as follows:

1. Please, go to Studio > Developer App > Forms > Data Items > System > Currency and 'Add a New Item' (new currency).

Example: If you would like to add the New Zealand dollar, please add:

List: Currency

Value: NZD

Title: __NZD (Important note: with two underscores)

Title Additional: (Leave this field empty)

Data:

a:1:{s:4:"sign";s:7:"NZ$";}

Note: NZ$ is the symbol code of New Zealand dollar (NZD). It is advised to enter the $ currency symbol as an html code like this:

NZ$

It's very important to enter the currency symbol as an html code. Otherwise it won't display in your site frontend.

In the data field, I used s:7 instead of s:6 or any other value because the html code for the New Zealand currency N Z & # 3 6 ; has 7-symbols length. If you use a different currency symbol (different html code for that currency), please change the s: value accordingly.

More details here: https://unacms.com/d/what-parameters-to-use-in-the-currency

Example: If you were to add, let's say, the Indian Rupee (INR), this step #1 would have been like this:

List: Currency

Value: INR

Title: __INR (Important note: with two underscores)

Title Additional: (Leave this field empty)

Data:

a:1:{s:4:"sign";s:7:"₹";}

Please note that I have used the html code for the Rupee ₹ symbol which is:

₹

Also I have used s:7 value in the data field because the the html code for the Rupee symbol & # 8 3 7 7 ; has 7-symbols length.

2. Then Go to the Polyglot App, create a new Key:

Key Name:__NZD (Important note: with two underscores)

Module: System

English: NZD

Russian: NZD

That's it. Your new currency will show in Studio > Settings App > System > General > Currency code (for checkout system).

3. To add a new currency in Studio > Payments > Settings > General > Currency (To be used during the payment process), 

Please, go to Studio > Developer > Forms > Data Items > Payments > Currencies

and add (Example for the New Zealand dollar):

List: Currency

Value: NZD

Title: NZD

Title Additional:

NZ$

Here as well (in the Title Additional field), enter the New Zealand dollar symbol as an html code.

Once again, example: If you were to add, let's say, the Indian Rupee (INR), the currency symbol to add here in the 'Title Additional' field would have been its html code which is:

₹

Data: (Leave this field empty)

4. Go to Studio > Settings App > System > General, in the Currency code (for checkout system) option, please select NZD, then in the 'Currency sign (for display purposes only) option, select the New Zealand Dollar symbol ⟨NZ$⟩ on the dropdown list.

5. In Studio > Payments App > Settings > Currency, please select NZD.

That's it! Hope this helps someone.