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 !