I need to know if it's possible to make other buildings or units to bunkers, someone said that it's hard coded, although you can make buildings able to load units.
But still such buildings or units that are able to load other units except for bunkers when they get destroyed everything inside follows.

The bunker is special because it loads units, allows them to attack from within and don't get destroyed when the bunker gets destroyed.
Can these functions be recreated or reused?

This is prolly crude as far as modding solutions go, but if your mod is just for UMS you can butcher an existing, unused unitID, and use EUD actions to form a new bunker

For example, change 0x59CD0C to 125 to change a unit at index 0 into a bunker

Unfortunately I'm looking for a modding solution to this problem, your suggestion will be useful, at least in mission maps where an other bunker would be used.
But could there be a modding solution to this, involving plugins maybe to replicate the behavior of the Bunker?

Afaik, Bunkers are extremely hardcoded. There may be a way though? What's different about the new Bunker you want compared to the old one? Depending on what you have in mind, you could possibly unit ID swap the new Bunker to the actual Bunker's unit ID via plugin and make the other changes happen in a manner depending on what they are.

Technically it's possible. Just find every reference to the Bunker's unit ID in the assembly code, and add a check for the second quasi-bunker. I assume there would be at least four places where the game checks the unit ID:

(1) Creating the unit
(2) Enter/exit transport
(3) Auto-attack
(4) Unit death (don't remove units previously loaded in bunkers)

The real problem is: how are you going to analyze the massive assembly code, let alone hack it?

You would also need to modify units.dat [transport capacity] and the button set for the quasi-bunker.

This conversation seems to have the guide lines, Thanks Pastelmind.