How to make and display a custom (seventh) need bar?

I want to make a thirst bar in the game, just like the hunger bar. It becomes red when drop low, and will cause the sim to die if it becomes zero. 

Now, the problem is, while adding the need is simple - just by statistic tuning, the "bar" is difficult. 

My experiment begins on the plant sim. I simply removed the blacklist of the "Bladder" need, and I created a sim with seven needs, all the reactions work as intended. Yet, the needs panel only shows 6 of them, and will just drop the last bar. If I make "Water" no.0, then "Hygiene" will not show up. If I make "Water" no.7, then "Water" will not show up. (Although they all work)

The problem now is, how can I show all seven of them? That is, I want this panel: (See P1, this is photoshopped)

I thought the thing could be automatically extended, and obviously it didn't work. In fact, I checked a lot of mods and all the DLCs, and found none that could extend the needs bar, only shorten them. - But the box could automatically shorten if I reduce the needs. (See P2) This is very strange. 

Well, I then checked the ui.package in the game file, and extracted the motive panel GFX. This looks like the core of the motive panel, and as I open them in JPEXS, it looks like this: (See P3)

It seems like the problem is that the game does not automatically make the bars, but make them in a static GFX file. Therefore, the game doesn't know at all how to display the seventh bar. 

Now, as I try to edit this panel, I found it really difficult. JPEXS doesn't have edit, and I just can't open it with any other software. I can make it a fla, but I don't know how the fla is made back into gfx. Also, I don't know if it would work if I just copy and paste the boxes. 

I just found a new thing few minutes ago, saying "SimInfoMotivePanelMain". Is this the code directing the panel? Would it work if I edit this? 

Well, can you help me make a new bar, like give some suggestions on where I should edit? Or, would you add the interface for more bars? This would be very helpful for mod creators. 

[BTW: I know that there actually exists a thirst motive, but I just want it to show up as a real motive. ]

+ After I was asking this question around, someone gave me this mod: https://modthesims.info/d/607604/automatic-sunburns-v10.html and it actually made the seventh bar (vitamin/skin, see P4). Therefore this should be possible. but how?