Sure! You can add a subject to your membership at any time. The cost will be calculated pro rata if you update your membership in the middle of a billing cycle.
To add a subject, sign in to your IXL parent account, and open your account menu. Select Membership details. In the Membership information section next to your current subject, click add English (or add maths) and follow the prompts to confirm your plan update. The additional subject will be available immediately!