How to append to the end of a line before the first pattern match?

I have a file containing some random data like:

number1 number2 number3
&END

I want to add a \ to the end of the line containing number3, but I cannot match a pattern on that line as that number keeps changing. So I try matching "&END" in sed and then append a line before it, but that does not work for me because it creates a new line, like

number1 number2 number3

\

&END

My desired output is

number1 number2 number3 \
&END

Also, I only need to make this change at the first occurence of &END in the file. Is there a way to do this using sed, or maybe awk?

EDIT: My input file is test.restart, that looks like:

&COORD

1.4089452021105357E+01  1.3165670625576730E+01  1.2727066323166799E+01 \
 ...

 ...

1.3549295360587577E+01  1.2120902691780184E+01  1.2741652733169291E+01

&END
...
...
&COORD

H 3475843 73457834 7346587435
...
...
&END

I only need to make the changes in the block of text before the first occurence of &END.

You can use sed -z and include \n in your pattern:

-z, --null-data
separate lines by NUL characters

sed -Ez 's/(\n+&END)/ \\\1/' file

sed by default replaces only the first occurency per line. As you have only one line if you use NUL-delimiter, you're fine.

-E tells sed to use Extended Regex (ERE) instead of Basic Regex (BRE). You can omit the flag, but then you need to escape the braces:

sed -z 's/\(\n+&END\)/ \\\1/' file

Output:

&COORD

1.4089452021105357E+01  1.3165670625576730E+01  1.2727066323166799E+01 \
 ...

 ...

1.3549295360587577E+01  1.2120902691780184E+01  1.2741652733169291E+01 \

&END
...
...
&COORD

H 3475843 73457834 7346587435
...
...
&END

Note, -z is fine in Ubuntu, but not available in all sed implementations and thus not a portable solution.