Sorry, Float2String is a homemade function.
I made a textfile with the name 18001_EN.hsl and put the following text in the file:

5000|"Houseofdreams Webdesign\Float To String 2 dec"|0|1|"E1 Float"|1|"A1 Text"
5001|1|1|0|0|0

5002|1|0

5004|1|""|0|1|1

5012|0|""|"'%.2f' % EN[1]"|""|1|0|0|0

Save the text in the directory "logic".
Mine is in c:\users\public\public documents\HS+FS Experte 2.7\logic
It's a directory with all the logic function blocks and all the files end with .hsl
Now the function will appear in the graphic logic editor after restarting Expert 2.7
Got this information from a user on a Belgian KNX forum.