Can I make a one-time investment even if I've set-up an SIP?

Yes, you can make a one-time investment (lumpsum investment) even if you've set up AutoPay for a SIP. To do so,

1. Tap Wealth at the bottom of your PhonePe app home screen.
2. Tap My Portfolio at the top of the screen. 
3. Select a fund under the Invested Funds section.
4. Tap Invest More.
5. Select One-time.
6. Enter the amount and tap Invest Now.