Diminutive forms are usually used for people or things, and they express smallness, affection, or familiarity. Since "will" is a verb, it doesn't naturally lend itself to a diminutive form.
However, you could use a more informal synonym like "want" or "wish" to convey a sense of desire or intention, which might be considered similar to a diminutive.
For example:
* Instead of: "He will go to the store."
* You could say: "He wants to go to the store." or "He wishes to go to the store."