Is it ok to stretch a steel fork from 90mm to 100mm?
You can spread steel forks safely (never spread aluminum/carbon fiber), and its not a trivial task to do at home, since you need to keep the fork blades spaced symmetrically and you don't have the same type of leverage available for the rear triangle. Then re-bend the axle mounts, as Criggie points out in the comments. 

Sheldon Brown has some tips on how to do it (pull the fork apart, check for alignment, use a rubber mallet when necessary), but I'd leave it to a shop if you can find one that can do it.