For what kind of usage? Personal?
Are you planning to start with the outlines of an existing digital font? If yes, from which foundry?

It is legal only if permitted by the End-User License Agreement (EULA) for the font. What a EULA allows for varies from foundry to foundry, but typically modification of an existing font is prohibited, especially if you’re planning to make money from the end result.

Maybe someone else can back me up on this.

What does the EULA say?

Its for commercial use. And yes I am starting with the outlines of Bodoni Poster Italic. I cant remember which foundry I got it from, so I dont know where to find the EULA. Is that usually packaged with the font? or is it something on their website?

The vendor name will normally be viewable within your Operating system - for example in Windows double clicking on a font will provide a screen that shows you the copyright string in the font.

Cheers, Si