Hello @all. I don't know hot to ask what I really need. But I try:

I managed to make own fonts with FontForge.
Their is a function to manipulate pairs of characters= kerning

What I need to know, if its possible with any software or any trick to manipulate more than character pairs.
I just need one specific case: the number "100"!
I wanna make the number "100" the same size as any 2 digit number, therefore I need my own character or glyph for the number 100.
Or maybe a trick, to change the "100" with any other unicode I don't need, e.g."&"...so instead of showing "100" it shows "&".
From their on I can manipulate the "&".
Its just a simple Font which I created with only numbers.
I am open for suggestions ...thx at all.

Build a 'ligature' of your '100' combined glyph by cutting, pasting and scaling and park it some where easy to remember in day to day use is the easiest way to do this. Creating a proper ligature, so when you type 100 there is a dlig lookup that automatically spots the (three glyph) 100 and substitutes the single reduced width '100' single glyph is the other way. Seemsa a fairly comprehensive Tut here for creating ligatures.
https://design.tutsplus.com/tutorials/how-to-design-and-construct-a-functional-cursive-font--cms-23295