It is possible to mod them yourself fairly easily to be not only playable, but playable and balanced. They'll just end up lacking many flavor units, and have less variety in leaders. Almost all have at least one unique national unit already though, but for the unique terrain improvement I tend to use one that's used by a neighbor or a culturally close civ. Like the Spanish mineria for Portuguese also.
To do this involves modifying various files under Realism > Assets > XML folder. You only need a text editor (but do make a back-up before you do anything).
First, under said folder, there is a folder labeled Civilizations and in it various files whose names are self-explanatory. As an example, I'll make the Portuguese playable.
It's under derivative civilizations, so I'll open that file and find their entry. Change the value in <bPlayable> from 0 to 1 and for <bAIPlayable> as well, so the AIs can choose it too. Technically you're already good to go. You can now start a game as Portugal. You can do this for every single civ you find under derivative and other civs folders. But the issues Tabriz mentioned need to be addressed for a balanced (or even half-way sane!) game, especially on a random map.
Joao's traits are imperialist, expansionist and reckless, which was an old trait name that now means either revolutionary or schemer, can't remember which. Those are normal traits, so no fixing needed here. But for example the Khmer leaders have "semi-civilized" which is a super-negative trait and needs to be changed to something ordinary before there's any point in using them. The leaders are found under the leaderheads file, obviously.
To fix the settler problem: Comment out (erase)<UnitType>UNIT_SETTLER_EUROPEAN_MINOR</UnitType>.
This way they build the default settler, which doesn't require a prereq tech.
To give the Portuguese a unique terrain improvement, the quick way is to give them access to the spanish worker by writing SPAIN in place of PORTUGAL in this line:
<UnitType>UNIT_WORKER_PORTUGAL</UnitType>.
It's not the "proper" way to do it, though. You should actually find the unit somewhere in the appropriate units file, and replace the list of its actions by those of the spanish worker. Functionally no different, but you'll have the correctly named unit then.
Next -- add the following lines to the building list
<Building>
<BuildingClassType>BUILDINGCLASS_LOCAL_BRONZE</BuildingClassType>
<BuildingType>NONE</BuildingType>
</Building>
Do the same for LOCAL_HORSES and LOCAL_IRON. These buildings allow the single-city civs Portugal, Israel and Netherlands to have access to resources they normally couldn't get except by conquest or trading. The NONE line disables them.
Up next. Ever wonder how those city states tend to have marvellous early game science output? It's because their palace is on steroids. You need to remove the lines about PALACE_CITY_STATE, which mean they just get the default palace.
At this point, if my memory serves, you're ready to go try your first game as Portugal. They have a decent unique building already in Feitoria, and I think they had a national unit also.
The process isn't quite the same for all the civs though.
There's three types I'd bother modding into playable:
1. The city states, Portugal, Israel, and Dutch, for which I gave an example of the procedure.
2. The derivative "vassal" civs, like Mexico, Texas, Finland... These are the easiest as they only lack a unique terrain improvement. Just decide what they should get, if any. Note: it's tempting to try and create your own. But that is a whole other ball game. With these methods you can only stick to modifying what already exists and who gets to build it. Adding stuff is trickier. Also Nubia and Babylon fit this list.
3. The more developed "semi-civilized" civs. Khmer, specifically. The Berbers as well I suppose (their unique building is bonkers last I looked.) These need to have their leaders negative trait modified. They also can't build workers (iirc) immediately, rather they rely on foragers, which you need to change.
This isn't comprehensive by any means, just a suggestion on how to get going. I may have forgotten something important, but it should be simple to figure stuff out once you just start editing the files. Cross checking the lines of the civ you're modding to an existing civ's is the best way forward, usually. There's also plenty of rulers that are used in scenarios. It's fun adding them to the list of playable rulers for a civ. (Many rulers are so obscure that you'll need to use wikipedia to figure out which civ they hail from.)
