I don't understand why you first say:
yeah i've already got all my custom callsigns setup with an addon and saying stuff with sideradio
And then you say the exact opposite:
is there anyway of making your own set of those, b/c i know the game allows you to assign "alpha", "bravo", "charlie", "november" ect...
i want to be able to assign a custom callsign...
Like I said before, it is very possible to define your own radio callsigns via an addon; just like in the Tonal addon, for example. I'm not sure off the top of my head how to do it, but if you look in the official config, you can find where BIS defined the standard callsigns. Then you can just copy and paste that section into the config for your own addon, and change the names from "alpha"/"bravo"/etc to whatever you want.
However, if you don't want to make a dinky little addon just to give you custom callsigns, you can either (a) make your mission on Tonal, or (b) try to use custom "HQ" callsigns. Other than that, you are outta luck
then also instead of defining each radio message in a stringtable, you'd be able to use sidechat and make them up in the mission editor "on the run" so to speak...
You NEVER have to define a radio message in the stringtable. Wherever you would put a reference to the string table, you can instead just use a string in quotes. However, the stringtable is handy because it gives you a SINGLE location you can go to change the radio text used in your mission, instead of having to search through all your triggers/scripts/etc if you want to change the text.