Here we are with a novel tutorial, on how to add a portable PC in the START menu, which conveniently has just the space for one more voice :D I made it so that it appears together with the POKéDEX voice right after we get the, well, POKéDEX item, as I didn't feel like at all handling any weird interaction for the player having or not having a Pokémon. Of course it can be made available only after the player get the first mon, as it is the POKéMON voice itself.

Anyway, we need to edit just four files.

Add the new start voice function

In engine/menus/start_sub_menus.asm, add the following:

StartMenu_PortablePC:: ; new
	ld a, [wCurMap] ; we don't want to cheese the Elite4, do we?
	cp LORELEIS_ROOM
	jr z, .cantUseItHere
	cp BRUNOS_ROOM
	jr z, .cantUseItHere
	cp AGATHAS_ROOM
	jr z, .cantUseItHere
	cp LANCES_ROOM
	jr z, .cantUseItHere
; if none of the above cp is met, let's open the pc and do the things
	callfar ActivatePC ; main part
	jr .done
.cantUseItHere ; no cheese!
	ld hl, CantUsePCHere
	call PrintText
.done
	call LoadScreenTilesFromBuffer2 ; restore saved screen
	call LoadTextBoxTilePatterns
	call UpdateSprites
	jp RedisplayStartMenu

CantUsePCHere:
	text_far _CantUsePCHere
	text_end

Add the new text for when opening the PC is not allowed

In data/text/text_2.asm (or really, in any file, for what matters, but here it made the most sense to me), add the following:

_CantUsePCHere:: ; new, for portablePC
	text "You cannot use"
	line "this here!"
	prompt

(yes, prompt and not done, otherwise it will autoclose immediately)

Edit the graphic of the START menu to include the new voice

We need to edit engine/menus/draw_start_menu.asm in the following ways:

DrawStartMenu::
	CheckEvent EVENT_GOT_POKEDEX
; menu with pokedex + portablePC
	hlcoord 10, 0
-	lb bc, 14, 8
+	lb bc, 16, 8 ; edited for portablePC
	jr nz, .drawTextBoxBorder
; shorter menu if the player doesn't have the pokedex + portablePC
	hlcoord 10, 0
	lb bc, 12, 8
.drawTextBoxBorder

[...]

CheckEvent EVENT_GOT_POKEDEX
; case for not having pokedex + portablePC
	ld a, $06
	jr z, .storeMenuItemCount
; case for having pokedex + portablePC
	ld de, StartMenuPokedexText
	call PrintStartMenuItem
-	ld a, $07
+	ld a, $08 ; edited for portablePC
.storeMenuItemCount

[...]

.printSaveOrResetText
	call PrintStartMenuItem
	ld de, StartMenuOptionText
	call PrintStartMenuItem
+	CheckEvent EVENT_GOT_POKEDEX ; new, for portablePC
+	jr z, .dontPrintPortablePC ; new, for portablePC
+	ld de, StartMenuPortablePCText ; new, for portablePC
+	call PrintStartMenuItem ; new, for portablePC
+.dontPrintPortablePC ; new, for portablePC
+	ld de, StartMenuExitText
	call PlaceString

[...]

StartMenuOptionText:
	db "OPTION@"

+StartMenuPortablePCText: ; new
+	db "PORT.PC@"

PrintStartMenuItem:

[...]

Edit how the START menu behaves

Finally, edit home/start_menu.asm, as it follows:

[...]

; if the player pressed tried to go past the top item, wrap around to the bottom
	CheckEvent EVENT_GOT_POKEDEX
-	ld a, 6 ; there are 7 menu items with the pokedex, so the max index is 6
+	ld a, 7 ; edited; there are 8 menu items with the pokedex + portablePC, so the max index is 7
	jr nz, .wrapMenuItemId
-	dec a ; there are only 6 menu items without the pokedex
+	ld a, 5 ; there are only 6 menu items without the pokedex + portablePC
.wrapMenuItemId

[...]

; if the player pressed tried to go past the bottom item, wrap around to the top
	CheckEvent EVENT_GOT_POKEDEX
	ld a, [wCurrentMenuItem]
-	ld c, 7 ; there are 7 menu items with the pokedex
+	ld c, 8 ; edited, there are 8 menu items with the pokedex + portablePC
	jr nz, .checkIfPastBottom
-	dec c ; there are only 6 menu items without the pokedex
+	ld c, 6 ; edited, there are only 6 menu items without the pokedex + portablePC
.checkIfPastBottom

[...]

	CheckEvent EVENT_GOT_POKEDEX
	ld a, [wCurrentMenuItem]
	jr nz, .displayMenuItem
-	inc a ; adjust position to account for missing pokedex menu item
+	inc a ; adjust position to account for missing pokedex menu item; from my understanding and testings this can stay the same, but please check
.displayMenuItem
	cp 0
	jp z, StartMenu_Pokedex
	cp 1
	jp z, StartMenu_Pokemon
	cp 2
	jp z, StartMenu_Item
	cp 3
	jp z, StartMenu_TrainerInfo
	cp 4
	jp z, StartMenu_SaveReset
	cp 5
	jp z, StartMenu_Option
+	cp 6 ; new
+	jp z, StartMenu_PortablePC ; new

; EXIT falls through to here

[...]

And we're done! Enjoy your portable PC! :)