To expound mark's answer:
The sample EmbedFontPostFacto.java (EmbedFontPostFacto.cs) from chapter 16 of iText in Action — 2nd Edition shows how you can embed a given True Type font in an existing PDF.
Be aware, though, that certain assumptions are made here, it's just an example after all. When generalizing the code for generic PDFs and fonts, the font dictionary should be checked more thoroughly and embedding the font file can require slightly different entries to be changed. In that case let the specification ISO 32000-1:2008 (especially chapter 9 Text) be your guide.