Waffen-SS DIV != Waffen-SS DIV.
I guess all know that by now.
But the whole topic is very "tricky" to just claim all were "bad" or all where "elite".
One point about SS Divisions many forget is, that they were all volunteers up to late in the war. Just because SS was not allowed to draft, just because that was the realm of Wehrmacht.
(Deal made with Nazi party in the years before, SA..)
So as recruits were harder to get by with th eongoing war, the SS recruited whereever they could get people. A part that Wehrmacht would of course not do.
Waffen-SS DIV also did not all get the newest stuff first, but diverted different what they got. The first units brought up their own gear like motor bikes and weapons from WW1 to figtht.. Thats more like Militia as an Army formation. That changed later of course a bit. The Partisan fighting units were not that good equiped at all too. And Waffen-SS did try to resue all they can get too boost their units. CoH player might remember the Hotchkis-tank. What an "über-tank" for the Waffen-SS..

Some DIV got better training and also weapons. And the "better" SS-Divs also where bigger in manpower as usual Wehrmacht on paper.
To understand SS-Div better,
people should not forget that the SS was a "state in the state" and Himmler was after getting an own territory in Europe as core SS-country.
The
SS was constantly fighting for ressources/power within the nazi Reich. And in many records of
Wehrmacht officiers you can read about the dislike of the Waffen SS as the fighting branch of that "state/organisation" SS. After all the Wehrmacht made the deal with the nazi party to obey Hitler in case the SA will de dropped back in the days(
https://en.wikipedia.org/wiki/Night_of_the_Long_Knives), and now they had nearly the another military organisation next to them with the SS. But and maybe because of that, they used the better SS-Divs many times for dangerous tasks on the battlefield. Because of the special status, Waffen-SS didn't needed to obey Wehrmacht command if they didn't wanted. There was noting a Wehrmacht General could do about it expect to ask Himmler or Hitler. Most times they followed the orders of Wehrmacht though.
As a rule of thumb, you could say that from the battle reports, generally the formations with better equipment like Panzer, Mot/Mech, were the better units of the Waffen-SS.
But other smaller formation fought also very brave, while others clearly did not.
To have foreign volunteers in the game, it could be like before with exp. forces. E. g. in the case of India for UK with an ingame mechanic.
Spanish Legion is a good example too. Or the many other units that fought for the UK.
Events could create them aswell if HoI4 is not very much different from HoI3 here. So I see no problem here. Same for the various foreign Waffen-SS Divs, can alle be created via events and also likely changed via events.
In case of foreign people fighting for others, this would be revolt/partisan mechanic.
Defecting to Axis/Allies soil to form units, that was so rare that an event chain could do it. How much would you create that way anyway? Like we see in history, even the volunters of Waffen-SS were most times not that numerous to form regular DIV with 15k+ soldiers like the Wiking-Div.
I can understand that a button somwhere, that is called " ask for volunteers" might be nice, but I doubt that we'll get such.
Easiest "modding" would maybe an event chain or adding a national focus wich triggers the search for volunteers.
"Activate the Gurkhas" national focus for UK.. Around 250k men dedicated to fight extra..