If you were playing with BGT or Easy TuTu you could do it with TobEx.
I don't know how you could do it with the vanilla BG1.

It can be done relatively easily with Near Infinity, but NI's interface can take some getting used to. (The UI isn't very user-friendly). It's trickier in BG2/Tutu/BGT because the proficiencies are added as effects, but in base BG1 you just open up the save, open up your character, and set however many stars you'd like.

I don't really think you can, because it's more an UI "bug". You should hack the game itself, not the character, so it will be very difficult, if possible. I recommend you to play with a BGT like mod, because in BG 2, you can have the 2 slots by default.

Short answer is yes you can.

The issue is one of slots... when you look at your character controls in game, you have weapon, find traps, thieving, stealth and cast spell. Which ability do you want to lose in order to get a second weapon slot?

Mods; there are already mods around that remove one (normally thieving) and place a new icon in the special ability i.e. the star (F12) at the end... you can see an example of this type of work around in the vanilla game by just taking Tiax... as a Thief/ Cleric he uses the slot for his ‘Turn’ un-dead.

Since you mention Gatekeeper, I'll assume you're talking about vanilla BG1. I'd recommend Sword Coast Keeper instead. It's user interface is simple, and it's more mod friendly than Gatekeeper (upon which it's based). Once you set the BG1 installation directory and restart the program, open the game you've already started, go to the Proficiencies tab and change your weapon skills as required. When done it will save as a new saved game...