I tried to modify a copy of cmsy10, by renaming it. My aim is to insert a external glyph from another font family (e.g euler). With fontforge tools, I created the font and inserted it into my local tree. When I tried to use it (in Plain TeX) the log said that there weren't sufficient symbols.
I've inserted correctly the new font and created a map file too. In addition, TeX seems to recognize it because it creates correctly its font table. However, it does not work as symbol font family.
Right now, I think there are two possible aspects: I did something wrong during the customization with fontforge, or TeX does not allow so easily to insert a new font for the symbols.
Is it right?

+come from cm and+come from euler. – David Carlisle Dec 01 '13 at 19:25pfb~ andafmfiles that I turned intotfmone withafm2tfmutility. Thus, I insert these two new font (pfb and tfm) into my local tree and tried to use it (with the subsitution of\tensy` font). – Martino Dec 01 '13 at 21:54\char#instruction? However, my aim is to create a font the allows me to compile my.texfiles without change or add anything. – Martino Dec 01 '13 at 21:56acomes from cmr-comes from cmsy and+comes from euler then$a+a-a$will use the specified fonts. – David Carlisle Dec 01 '13 at 22:10