And, if you need the original number to be a var, put it in a var and use str_for_num( string , var )
This will convert your var to string, and then use 3run's example to add to the string and convert back to var...

If you want to use numbers only (no strings) you could multiply the first number by the next round big of the second number, in your example:
10 * 100 = 1000 + 23 = 1023.
Another example:
432 and 569
432 * 1000 = 432000 + 569 = 432569...