reserving space in a rec for future update

Hi:

I have a situation where I will be inserting thousands of records into a table but leaving 2 of it’s columns null.  Later on, I will be updating most of those records and putting real values in  place of those 2 nulls. As for the ones that do not get updated, I want to leave them null.   My concern has to do with record fragmentation at the time of update because there’s no room to “expand” them to accept the non-null data.  (BTW, the columns are floating point).

Is there a way to initially insert nulls, but reserve space for the future update (and avoid record fragmentation)? 

Is my record fragmentation concern unfounded?

Thanks in Advance !