Can I contribute additional amounts if I want to buy additional shares of stock?
Some companies may allow this, but the practice is not common. When you purchase ESPP shares and they remain in your account, dividends paid by your company may be reinvested in more shares, but not at a discount. Check your plan for your company's policy.
See a related FAQ about the maximum amount, under both the plan and IRS rules, that you can contribute from your paycheck.