I googled your question and found a couple links that might be of some help to you.
I believe you have to open one letter in a window, then you have to open the second letter in a new window;now choose "select design to move", right click, choose "copy". Go to the window where the first letter is click "paste"- the second letter should appear on top of the first, so move it around and click "apply". Good luck.
Have you tried copy and paste? My machine was Jaguar, similar to singer futura, and copy and paste worked for me. Hugs,Yoriko
I'm sorry, I don't have that machine but I will send your question back to the top. Are you trying to do built in lettering or fonts you get from the internet?