Is it possible to make my own ligatures?

I don't think this is possible, since ligatures are a feature of the font.

You can imitate some of these ligatures by adjusting the kerning with, for instance, the technique Ulrike Fischer explains at https://tex.stackexchange.com/a/312160/. E.g.,

documentclass{article}
usepackage{fontspec}
directlua{
 fonts.handlers.otf.addfeature{
    name = "newlig",
    type = "kern",
    data = {
      ["O"] = { ["O"] =  -450 , ["C"] =  -450 , ["G"] =  -450 },
    },
  }
}
setmainfont{CMU Serif}
begin{document}
BOOK OC OG

{addfontfeature{RawFeature=+newlig}
  BOOK OC OG}

tt tkern -.13em t
end{document}

However, the result is crowded and displeasing, because the O of Computer Modern is narrow and the bottom of the t curves up steeply. The ligatures which interest you have their natural home amid the more spacious proportions of a Renaissance typeface. Because the character of the whole typeface, not individual glyphs only, needs to be considered when adding a ligature, the project is best left to professional type designers.