It is okay to introduce the high chair even if your baby is only doing supported sitting. You can start with short sessions of about 5 minutes, twice a day, to help them familiarize themselves with the posture and the environment. Ensure their hips are positioned well back, their back is fully supported, and they have a stable footrest. Always supervise them closely, and if you notice them slumping to one side, continue practicing supported sitting on the floor until they develop more trunk strength.