Can I create a custom attribute that has an enumeration but also allows an integer value?

When you click on the triangle by Effective Estimate, you get an enumeration of different values that you can select from.  You can also click on the text of Effective Estimate and get fields where you can type in values different than what is on the Enumeration list.  I'd like to be able to do a similar thing with Story Points (Complexity)... where it will give me the standard list of Fibonacci type numbers, but would also allow me to hand modify the numbers.  The use case for this is, I want to pre-estimate the size of an epic with a place holder story... say 100 pts.  But, then I want to create "real" stories, say one at 5 pts and one at 8 pts... So, to maintain the epic at 100 pts, I'd need my place holder story to be worth 87 pts... maybe there is a different solution?