How to assign several characters to one key-stroke in xkb?

Question

We are using a custom written (by me) xkb Symbols file for a minority language. This has worked fine for over ten years and we roughly know how the system works. I am presently re-working the keyboard as the language is evolving.

Now I noticed we have a few positions left on our layout (Marked NoSymbol) and would like to assign very frequently needed "strings" of several characters. I would like to do this at the level of xkb so that we have consistent behaviour in our entire office, and can help other users, whatever the Linux distributions and versions, and different hardware keyboards.

I have searched through many posts here and outside StackExchange and it seems nobody ever covered this need or I am using bad key words. I have also once custom-written an Android keyboard for the same minority language and could easily assign text-strings to the "leftover keys".

I have tried guessing the syntax for our Symbols file, with "quotes" and without quotes and got no joy. I also tried whether I could assign this in our keysymdef.h but noticed that it is absent from the old, known location at /usr/include/x11/keysymdef.h and cannot find any new location in OpenSuse Leap 15.2.

This is an example line, where I would like to add the string "it works" in the third position (to facilitate communication):

key <KPDL>  { [   comma,  period, NoSymbol, NoSymbol  ] };                                     

Thank you for any hints.