Custom Function from worksheet result

If you already have the formula setup in A2 to return a result based on A1, why not just fix the references to all cells except A1 and drag it over as far as you need? Then B1 on could be the numbers you want to return the values of.
I'm not aware of a way to get your own custom function in without code. You can have formulas that are shortened to look like custom formulas by using the name manager tool, but you can't create one with an input.
For your last question, there's no way we can really answer that without knowing what you're trying to accomplish. Can you please provide an example workbook of what you're trying to do (including an explanation of how the other variables would affect the formula) along with expected results?

Maybe what I need is actually a macro.
Check this file I attach.
Give a number in A1 and it returns the result in A2
The actual workbook is more complicated. I made this as a simple example of what I am trying to do.
What I want is to create a function that puts the input variable in A1, reads A2 and returns it as result.
In that way I can construct a table having 1,2,3,4,5,6,7,8,9,10 on the first column and the appropriate A2 values on the second. I can do that by using multiple instances of the C1:G9 table and dragging the required cells one under the other. The final workbook though is much more complicated and it would really be painful to work that way.