Okay maybe I was confused by what you are asking for.
Are you asking for a 'buff/debuff' in which the percentage gains stack (i.e. first application adds .25% increase, second application adds an additional .25%, etc.)?
If this is the case you could use a state that adds each percentage gain (i.e. state 1 raises the stat by 1.25%, state 2 raises the stat by 1.5%, etc) separately; then, when the next tier of state is applied the last one is removed.
On the states tab, under traits, you can add a % boost to a parameter.
If you use Yanfly Skill Core, you could use lunatic code:
In the code portion, you could use an if/else statement to check if target is affected by a state. In this way, you could check if the target has the first tier state applied and if not add tier 1 state; if tier 1 state is applied then remove it and add tier 2 state; etc.
Is this perhaps more along the lines of what you are wanting to do?