Add data dynamically to Next Step

[mitsus]

Hi,
my question is...is possible to add data dynamically to an Next Step?
That is i've first step with two buttons. When i click first button i want load some data into next step, if i click second button i want see others data.
I tried, but when it load steps give an NullPoiterException (in second step) because data are not available in first istance, but only when i click on buttons into first step.

Thanls a lot