A slight variation on the fontedit suggestion is to make a second version of the font to hold your missing characters.
For example, you could write 'n' in the standard Decker body font and then switch it over to body2 to make it appear as 'ñ' instead.
Edit: Brainstorming a sillier way...
As long as you're only using ñ inside of 'rich text' fields you can use an image of the letter.
%%WGT0{"w":[{"name":"lettertest","type":"canvas","size":[5,12],"pos":[206,149],"locked":1,"border":0,"image":"%%IMG0AAUADABQoADgkJCQkAAAAA==","scale":1}],"d":{}}
If you paste this into your deck it will make a small canvas. You can "Edit > Copy Image" and then paste it while editing a text field when you need an ñ. This was made to match the 'body' font.
Because it's an image it doesn't work exactly the same way as letters do. It's fine with 'solid' fields, can work with transparency if it's prepared right and it doesn't really work with 'inverted' -- you'd have to make an separate inverted image for that.
You'd also need a separate little canvas for "Ñ" and any other letters you need so... I would generally recommend fontedit as the solution. Especially since it's your mother tongue . 🙂 But if you're ever missing something you need... you can sneak it in with an image.