Cannot be done. It'll always remain at 5. For 5 is the number it shall be. The number 5 being the amount of shield bars a Zoltan Shield Provides. For the number 5 is what you are stuck with. 6 also is not something that can be attained, nor is the number 4 unless it started at 5 and it took damage.
30 is right out.
Once the number 5 of the Zoltan Shield has been attained for that is the number it shall be, it can be used to protect thee from thy foes who be naughty in your sight shall snuff it.