The short answer is: yes, you can use indoor cushions on your balcony or patio furniture, but only with strict diligence and understanding of the risks. The core condition is your stated plan: you must bring them inside at the first sign of rain and whenever not in use.
Indoor cushions are not built for the outdoor environment. Their fabrics lack UV inhibitors and water-repellent treatments, and their fillings (like cotton or down) are highly absorbent. Even brief exposure to rain can lead to slow drying, mildew growth, and permanent stains. Sun exposure will rapidly fade colors and weaken fabrics.
If you proceed, follow this protocol. Use a weatherproof storage bench nearby for quick stash during sudden showers. Always fluff and air cushions indoors after use to prevent moisture buildup. Consider applying a fabric protector spray for a temporary moisture barrier. For a more carefree solution, invest in outdoor-rated cushions with quick-dry foam and solution-dyed acrylic fabric. They are designed to withstand sun and dry quickly after a light shower.
Ultimately, using indoor cushions outdoors is a high-maintenance choice. It works only if you are committed to the constant routine of moving and monitoring them. For lasting beauty and comfort, purpose-made outdoor cushions are the recommended investment.